Chapter III: Introduction (2)

I. Fossils displaying radial symmetry—symmetrical repetition of parts
around a central axis
A. Fossil tapering, cylindrical, cone-shaped:
1. Fossil with longitudinal radial partitions or septa;
cone-shaped
Coral
2. Shell with transverse septa or sutures; tapering at one end
Cephalopod
3. Shell without internal septa or partitions:
a. Shell large, heavy; usually with external longitudinal ribs.
Occur only in Cretaceous rocks
Rudistid
b. Shell small (usually less than 2 inches long), tusk-shaped,
open at both ends. Rare in Paleozoic and Mesozoic rocks
Scaphopod
B. Fossil disk-shaped or flattened dome-shaped:
1. Fossil with radiating star pattern on top
Echinoid
2. Fossil subconical to hemispherical, dome-shaped; base concave
or flat; minute pits or pores covering surface; typically
small (less than 3 inches across)
Bryozoa
3. Fossil small (less than ½ inch); generally disk-shaped
Foraminifera (orbitoidid)
4. Fossil disk-shaped or button-like; with longitudinal, radial
partitions or septa
Coral
C. Fossil composed of segments or plates:
1. Fossil composed of circular segments, disks, or chambers; when
united form cylinder:
a. Tapered shell
Cephalopod
b. Non-tapered, segments small and of relatively uniform
thickness with hole in center; individual columnals
disk-shaped
Crinoid stem
2. Fossil composed of many-sided plates:
a. Bud-shaped fossil of 13 wedge-shaped plates
Blastoid
b. Cup-shaped fossil of many curved plates surrounded by
branching arms
Crinoid
II. Fossils displaying bilateral symmetry—symmetrical duplication of
parts on each side of a plane
A. Fossil coiled in a single plane:
1. Shell divided by internal transverse partitions or sutures
Cephalopod
2. Shell without internal partitions or sutures
Gastropod
3. Shell small; spindle-shaped; resembles wheat grain. Common in
Pennsylvanian and Permian rocks
Foraminifera (fusulinids)
B. Fossil not coiled:
1. Shells or valves similar to clams:
a. Plane of symmetry parallel to hinge; equivalved
Pelecypod
b. Plane of symmetry (almost bilaterally symmetrical) at right
angles to hinge line; mostly inequivalved; strongly ribbed.
“Scallop-like” with “ears.” Rare in Paleozoic rocks
Pelecypod
c. Plane of symmetry at right angles to hinge line;
inequivalved; not “scallop-like” and without “ears.” Larger
valve commonly has an opening in beak. Common in Paleozoic
rocks
Brachiopod
2. Fossil tapering, cylindrical, cone-shaped:
a. Fossil with internal longitudinal, radial septa or
partitions; cone-shaped
Coral
b. Shell with internal transverse partitions or sutures;
tapering at one end
Cephalopod
c. Shell without internal septa or partitions.
(1) Shell large, heavy; usually with external longitudinal
ribs. Occur only in Cretaceous rocks
Rudistid
(2) Shell small (usually less than 2 inches), tusk-shaped,
open at both ends. Rare in Paleozoic and Mesozoic rocks
Scaphopod
3. Fossil heart-shaped, domed or flattened; radial star pattern on
top
Echinoid
4. Fossil segmented:
a. Fossil divided into 3 lobes; may be curled up. Not found in
Mesozoic or Cenozoic rocks
Trilobite
b. Fossil flattened or elongate; resembles shrimp, crab, or
crayfish
Crustacean
III. Fossils displaying no apparent symmetry
A. Shell without transverse internal partitions or sutures:
1. Shell coiled like ram’s horn, low spired, opening of shell very
large; surface has concentric ridges. Shell has two valves;
smaller, flattened valve not often found. In Texas found only
in Cretaceous rocks
Pelecypod
(Note: Some Paleozoic gastropods, “2,” closely resemble larger
valve of these pelecypods)
2. Shell tightly coiled; most have higher spire than “1.” Opening
of shell smaller than “1”; shell not as rough as “1” and has
only one valve
Gastropod
B. Coiled fossils; coiling not in one plane:
1. Shell with transverse internal partitions or sutures:
a. Partitions always smooth; thick shelled; loosely and
irregularly coiled, usually in large compact masses of many
individual shells. Occur only in Cretaceous rocks
Caprinid
b. Partitions (sutures) usually wrinkled; relatively thin
shelled; mostly regularly and tightly coiled; occur as
separate individual specimens
Cephalopod
2. Shell without transverse internal partitions or sutures
Gastropod
3. Solid spiral ridge around central axis; resembles a corkscrew
Bryozoa
C. Uncoiled fossils:
1. Fossil resembles a narrow saw blade; typically found as thin
film of carbon. Not found in Mesozoic or Cenozoic rocks
Graptolite
2. Fossil irregularly cone-shaped; longitudinal radial partitions
or septa
Coral
3. Shell resembles a clam or oyster shell but valve or shell not
symmetrical
Pelecypod (mostly oysters)
4. Branching twig-like fossils:
a. Fossils covered with minute pores or openings
Bryozoa
b. Fossils with evenly distributed, relatively large openings
with longitudinal radial partitions or septa
Colonial coral
5. Lace-like fossils; occur as thin sheets or films
Bryozoa
6. Fossils composed of radiating masses of polygonal or circular
tubes containing radial septa
Colonial coral
7. Irregular fossils; typically cylindrical with rough surface:
a. Fossil has large axial opening and thick wall; usually has
external longitudinal ribs. Occurs only in Cretaceous rocks
Rudistid
b. Fossil solid with no large axial opening; surface with small
pits or pores (fewer than in Bryozoa). In Texas, occurs most
commonly in Pennsylvanian and Permian rocks
Sponge

LIST OF TEXAS COLLEGES OFFERING GEOLOGY COURSES

A.&M. College of Texas, College Station
Amarillo College, Amarillo
Arlington State College, Arlington
Austin College, Sherman
Baylor University, Waco
Blinn College, Brenham
Corpus Christi, University of, Corpus Christi
Del Mar College, Corpus Christi
East Texas State College, Commerce
Hardin-Simmons University, Abilene
Henderson County Junior College, Athens
Houston, University of, Houston
Howard County Junior College, Big Spring
Kilgore College, Kilgore
Lamar State College of Technology, Beaumont
Lee College, Baytown
McMurry College, Abilene
Midwestern University, Wichita Falls
North Texas State College, Denton
Odessa College, Odessa
Pan American College, Edinburg
Rice University, Houston
St. Mary’s University, San Antonio
San Angelo College, San Angelo
San Antonio College, San Antonio
Southern Methodist University, Dallas
South Texas College, Houston
Southwestern University, Georgetown
Stephen F. Austin State College, Nacogdoches
Sul Ross State College, Alpine
Tarleton State College, Stephenville
Texarkana College, Texarkana
Texas Christian University, Fort Worth
Texas College, Tyler
Texas College of Arts and Industries, Kingsville
Texas Technological College, Lubbock
Texas Western College, El Paso
The University of Texas, Austin
Trinity University, San Antonio
Tyler Junior College, Tyler
West Texas State College, Canyon

A. Tapering, cylindrical cone-shaped fossils
1. Cone-shaped with longitudinal partitions or septa
Coral
2. Fossils with septa or sutures; tapering at one end
Cephalopod
3. Shell without internal partitions or sutures
a. Shell large heavy, external longitudinal ribs. Cretaceous only
Rudistid
b. Shell small, tusk-shaped open at both ends. Rare in Paleozoic
and Mesozoic
Scaphopod
B. Disc or dome-shaped fossils
1. Star pattern on top
Echinoid
2. Subconical small pits or pores on top
Bryozoan
3. Small disc-shaped (less than ½ inch)
Orbitoid Foraminifera
4. Disc-shaped or button-like, with longitudinal partitions or septa
Coral
C. Fossils composed of segments or plates
1. Circular discs or chambers; when united form cylinder
a. Tapered shell
Cephalopod
b. Not tapered, segments small of uniform thickness, hole in
center
Crinoid Stem
2. Fossil composed of many-sided plates
a. Bud-shaped, 13 wedge-shaped plates
Blastoid
b. Cup-shaped, many curved plates branching arms
Crinoid

A. Fossil coiled in a single plane
1. Shell divided by internal transverse partitions or sutures
Cephalopod
2. Shell without internal partitions or sutures
Gastropod
3. Shell small, spindle-shaped; resembles wheat grain. Pennsylvanian
and Permian
Foraminifera fusulinid
B. Fossil not coiled
1. Shells or valves similar to clams
a. Plane of symmetry parallel to hinge; equivalved
Pelecypod
b. Plane of symmetry almost at right angles to hinge; strongly
ribbed; “Scallop-like” with “ears”, inequivalved
Pelecypod
c. Plane of symmetry at right angles to hinge-line; without
“ears”, not “Scallop-like”; commonly with opening in beak,
inequivalved
Brachiopod
2. Fossil tapering, cylindrical or cone-shaped
a. Cone-shaped, internal longitudinal partitions or septa
Coral
b. Tapered, internal transverse partitions
Cephalopod
c. Shell without internal septa or partitions
(1.) Shell large heavy, longitudinal ribs. Cretaceous only
Rudistid
(2.) Shell small, tusk-shaped, open at both ends, rare in
Paleozoic and Mesozoic rocks
Scaphopod
3. Fossil heart-shaped, domed or flattened; star pattern on top
Echinoid
4. Fossil segmented
a. Divided into 3 lobes, may be curled up. Paleozoic only
Trilobite
b. Flattened or elongate, resembles shrimp
Crustacean

A. Shell without transverse partitions or sutures
1. Shell coiled like ram’s horn, low spired; shell has two valves,
smaller flattened valve often missing. In Texas exclusively
Cretaceous
Pelecypod
2. Shell tightly coiled, most have higher spire than 1, shell
smaller and not as rough as 1, has only one valve
Gastropod
B. Coiled fossils, coiling not in one plane
1. Shell with transverse internal partitions or sutures
a. Partitions always smooth, thick shelled, loosely and
irregularly coiled, in Texas exclusively Cretaceous
Caprinid
b. Partitions (sutures) generally wrinkled, regularly and tightly
coiled
Cephalopod
2. Shell without transverse internal partitions or sutures
Gastropod
3. Solid spiral ridge around central axis, resembles corkscrew
Bryozoan
C. Uncoiled fossils
1. Fossil resembles narrow saw blade. Paleozoic only
Graptolite
2. Fossil irregularly cone-shaped, longitudinal partitions or septa
Coral
3. Shell resembles clam or oyster, nonsymmetrical
Pelecypod (mostly oysters)
4. Branching twig-like fossils
a. Covered with minute pores or openings
Bryozoa
b. With evenly distributed larger openings with septa
Colonial coral
5. Lace-like fossils, occur as thin sheets or films
Bryozoa
6. Masses of circular or polygonal tubes with septa
Colonial coral
7. Irregular fossils, cylindrical with rough surface
a. Large axial opening with thick wall, external longitudinal
ribs. Cretaceous only
Rudistid
b. Solid, no opening, small pits or pores. Pennsylvanian or
Permian
Sponge

CATALOGING THE COLLECTION

After the fossils have been cleaned and tentatively identified, they should be cataloged. This is necessary to enable the collector to have a record of his collection and to furnish as much information as possible about each individual fossil.

The collecting data can be taken from the labels that were placed in each bag of fossils as they were collected, or from the field notebook. Actually, it is wise to check one against the other. This information should then be entered in some type of record book and also placed on a more permanent label which is put in the tray or box with the fossil. The catalog and label should contain such pertinent data as (1) the scientific name of the fossil, (2) the geologic formation from which the specimen was collected, (3) the exact geographic location of the collecting locality, (4) the name of the collector, (5) the date the fossil was collected, and (6) the catalog number of the specimen. The latter is usually placed in the upper right hand corner of the label (fig. 6) and corresponds with a like number in the record book.

Specimen No. P-185
NAME Spirifer rockymontanus
FORMATION Big Saline (Penn.)
LOCALITY Little Brady Creek, McCulloch Co., Tex.
(1000′ NE of Smith ranch House)
COLLECTOR F. B. Plummer
DATE July 1937

The entries in the catalog should be numbered consecutively, and all specimens from the same locality should bear the same number. This number should be written on the fossil with India ink, preferably on any remaining matrix or on some inconspicuous part of the specimen (fig. 6). If the surface of the fossil is too coarse or porous for ink, the catalog number can be written on a small patch of white enamel or clear nail polish painted on the specimen. After the ink has dried it should be coated with a dab of clear shellac or clear nail polish to help preserve the number. If each specimen is numbered, it can easily be identified even if it should become separated from its label.

HOW FOSSILS ARE USED

Fossils are useful in a number of different ways, for each specimen provides some information about when it lived, where it lived, and how it lived.

Fossils are very important, for example, in tracing the development of the plants and animals of our earth. This is possible because the fossils in the older rocks are usually primitive and relatively simple; but a study of similar specimens that lived in later geologic time shows that the fossils become progressively more complex and more advanced in the younger rocks.

Some fossils, for example, the reef-building corals, appear to have always lived under much the same conditions as they live today. Hence, it is reasonably certain that the rocks containing fossil reef corals found in place (that is, where they were originally buried), were deposited in warm, fairly shallow, salt water. By studying the occurrence and distribution of such marine fossils, it is possible to outline the location and extent of prehistoric seas. Moreover, the type of fossils present will frequently give some indication as to the bottom conditions, depth, temperature, and salinity of these ancient bodies of water.

Probably the most important use of fossils is for purposes of _correlation_—the process of demonstrating that certain rock layers are closely related to each other. By correlating or “matching” the beds containing specific fossils, it is possible to determine the distribution of geologic units of similar age. Some fossils have a very limited vertical or geologic range and a wide horizontal or geographic range. In other words, they lived but a relatively short period in geologic time but were rather widely distributed during their relatively short life. Such fossils are known as _index fossils_ or _guide fossils_ and are especially useful in correlation because they are normally only associated with rocks of one certain age.

Microfossils are often very valuable as guide fossils for the petroleum geologist. The micropaleontologist washes the well cuttings from the drill hole and separates the tiny fossils from the surrounding rocks. The specimens are then mounted on special slides (fig. 7) and studied under the microscope. Information derived from these fossils often provides valuable data on the age of the subsurface formation and the possibilities of oil production. Microfossils are particularly valuable in the oil fields of the Gulf Coast region of Texas. In fact, some of the oil-producing zones in this area have even been named for certain key genera of microfossils. For example, the “het” zone of Oligocene age (geologic time scale, Pl. 1) is named for the genus _Heterostegina_, which is a tiny one-celled animal. Other microfossils, such as fusulinids, ostracodes, spores, and pollens, are also used to identify subsurface formations in many other parts of the State.

Plant fossils are very useful as climatic indicators but are not too reliable for purposes of age determination. They do, however, provide much information about the development of plants throughout geologic time.

GEOLOGIC HISTORY

The geologic history of our earth has been recorded primarily in marine sedimentary rocks, and this record indicates that our earth is very old and that life has been present for many millions of years. The earth is not only extremely old (more than 3½ billion years of age), but it has also undergone many changes which have taken place slowly but steadily and have greatly affected both the earth and its inhabitants. The earth’s physical features have not always been as they are seen today. Geologic research has shown that mountains now occupy the sites of ancient seas, and that coal is being mined where swamps existed millions of years ago. Furthermore, there is much evidence to indicate that plants and animals have also undergone great change. The trend of this organic change is, in general, toward more complex and advanced forms of life, but some forms have remained virtually unchanged and others have become extinct.

In order to interpret geologic history, the earth scientist must attempt to gather evidence of the great changes in climate, geography, and life that took place in the geologic past. The record of these changes can be found in the rocks, and here is found the story of the various events in earth history.

GEOLOGIC COLUMN AND TIME SCALE

In order to discuss fossils and the age of the rocks containing them, it is necessary to become familiar with the _geologic column_ and the _geologic time scale_ (Pl. 1).

The _geologic column_ refers to the total succession of rocks, from the oldest to most recent, that are found either locally or in the entire earth. Thus, the geologic column of Texas includes all rock divisions known to be present in this State. By referring to the geologic column previously worked out for any given area, the geologist can determine what type of rocks he might expect to find in that particular region.

The _geologic time scale_ is composed of units which represent intervals of geologic time, during which were deposited the rocks represented in the geologic column. These time units are used by the geologist to date the events that have taken place in the geologic past.

The largest unit of geologic time is an era, and each era is divided into smaller time units called _periods_. A period of geologic time is divided into _epochs_, which, in turn, may be subdivided into still smaller units. The geologic time scale might be roughly compared to the calendar in which the year is divided into months, months into weeks, and weeks into days. Unlike years, however, geologic time units are arbitrary and of unequal duration, and the geologist cannot be positive about the exact length of time involved in each unit. The time scale does, however, provide a standard by which he can discuss the age of fossils and their surrounding rocks. By referring to the time scale it may be possible, for instance, to state that a certain event occurred during the Paleozoic era in the same sense that one might say that something happened during the American Revolution.

There are five eras of geologic time, and each has been given a name that is descriptive of the degree of life development that characterizes that era. Hence, Paleozoic means “ancient-life,” and the era was so named because of the relatively simple and ancient stage of life development.

The eras, a guide to their pronunciation, and the literal translation of each name is shown below.

Cenozoic (SEE-no-zo-ic)—“recent-life”
Mesozoic (MES-o-zo-ic)—“middle-life”
Paleozoic (PAY-lee-o-zo-ic)—“ancient-life”
Proterozoic (PRO-ter-o-zo-ic)—“primitive-life”
Archeozoic (AR-kee-o-zo-ic)—“beginning-life”

Archeozoic and Proterozoic rocks are commonly grouped together and referred to as Precambrian in age. The Precambrian rocks have been greatly contorted and metamorphosed, and the record of this portion of earth history is most difficult to interpret. Precambrian time represents that portion of geologic time from the beginning of earth history until the deposition of the earliest fossiliferous Cambrian strata. If the earth is as old as is believed, Precambrian time may represent as much as 85 percent of all geologic time.

The _oldest_ era is at the _bottom_ of the list because this part of geologic time transpired first and was then followed by the successively younger eras which are placed above it. Therefore, the geologic time scale is always read _from the bottom of the chart upward_. This is, of course, the order in which the various portions of geologic time occurred and during which the corresponding rocks were formed.

As mentioned above, each of the eras has been divided into periods, and most of these periods derive their names from the regions in which the rocks of each were first studied. For example, the Pennsylvanian rocks of North America were first studied in the State of Pennsylvania.

The Paleozoic era has been divided into seven periods of geologic time. With the oldest at the bottom of the list, these periods and the source of their names are:

Permian (PUR-me-un)—from the Province of Perm in Russia
Pennsylvanian (pen-sil-VAIN-yun)—from the State of Pennsylvania
Mississippian (miss-i-SIP-i-un)—from the Upper Mississippi Valley
Devonian (de-VO-ni-un)—from Devonshire, England
Silurian (si-LOO-ri-un)—for the Silures, an ancient tribe of Britain
Ordovician (or-doe-VISH-un)—for the Ordovices, an ancient tribe of
Britain
Cambrian (KAM-bri-un)—from the Latin word _Cambria_, meaning Wales

The Carboniferous period in Europe includes the Mississippian and Pennsylvanian periods of North America. Although this classification is no longer used in the United States, the term Carboniferous will be found in many of the earlier geological publications and on many of the earlier geologic maps.

The periods of the Mesozoic era and the source of their names are:

Cretaceous (cre-TAY-shus)—from the Latin word _creta_, meaning chalky
Jurassic (joo-RAS-ik)—from the Jura Mountains of Europe
Triassic (try-ASS-ik)—from the Latin word _triad_, meaning three

In Texas, the Cretaceous has two divisions, known as either Lower Cretaceous and Upper Cretaceous or as Comanche series and Gulf series, respectively. These designations are for rocks of nearly equivalent age, and both sets of terms have been used by geologists and in publications. In this handbook, both sets of terms are used interchangeably, that is, Lower Cretaceous and/or Comanche series and Upper Cretaceous and/or Gulf series.

The Cenozoic periods derived their names from an old outdated system of classification which divided all of the earth’s rocks into four groups. The two divisions listed below are the only names of this system which are still in use:

Quaternary (kwah-TUR-nuh-ri)
Tertiary (TUR-shi-ri)

While the units discussed above are the major divisions of geologic _time_, the geologist usually works with smaller units of _rocks_ called _formations_. A geologic formation is identified and established on the basis of definite physical and chemical characteristics of the rocks. Formations are usually given geographic names which are combined with the type of rock that makes up the bulk of the formation. For example, the Beaumont clay was named from clay deposits that are found in and around Beaumont, Texas.

THE GEOLOGY OF TEXAS

The geologic history of Texas, like the geologic history of the rest of the earth, is recorded primarily in marine sedimentary rocks. These rocks provide some knowledge of the early geography and the first inhabitants of what is now the State of Texas. Most of these rocks were formed from sediments deposited in shallow seas which covered parts of the State at various times in earth history.

By studying these rocks and their relations to each other, geologists have established a geologic column for Texas.

Physiography

In order to discuss the distribution and exposures of the rocks of Texas, it is helpful to be familiar with the _physiography_ of the State. Physiography deals with the study of the origin and description of land forms, such as mountains, valleys, and plains. Plate 9 is a map of Texas which shows the major physiographic provinces within the State.

The majority of the land forms in Texas have been produced by the processes of erosion attacking the structural features of an area. Certain other land forms may be related to the effects of igneous activity which resulted in the accumulation of large masses of igneous rocks. The Davis Mountains are an example of surface features produced in this manner.

In discussing the physiography of Texas, three major physiographic provinces will be recognized. These are (1) the Trans-Pecos region, (2) the Texas Plains, and (3) the Gulf Coastal Plain (Pl. 9).

TRANS-PECOS REGION

The Trans-Pecos region, located in the westernmost part of the State, is an area of mountains and plateaus with broad basins between the major mountain ranges. Many different types of rocks are exposed in Trans-Pecos Texas and these include marine, fresh-water, and terrestrial deposits. In many areas igneous rocks flowed out on the surface and now overlie sedimentary rocks. There are also many places where igneous rocks have been injected into the surrounding rocks, and these igneous rocks have been exposed by later erosion.

Included within this area is the Van Horn uplift of southern Hudspeth and Culberson counties, the Solitario uplift of southern Presidio and Brewster counties, and the Marathon uplift of northeast Brewster County. This region also includes the Big Bend area of Texas, a part of which has been set aside as a National Park where many interesting and important geological features may be seen.

The Trans-Pecos region is one of rugged topography with elevations as high as 8,700 feet, at Guadalupe Peak in the Guadalupe Mountains of northern Culberson County, and as low as 1,500 feet, in the Rio Grande valley.

Numerous invertebrate fossils occur in the Cretaceous limestones and shales of the Trans-Pecos region and in the Paleozoic rocks of the Marathon uplift. The Gaptank formation of Pennsylvanian age and the Permian reef limestones of the Glass Mountains are especially fossiliferous. In addition, many vertebrate fossils have been collected in Trans-Pecos Texas, particularly in and around Big Bend National Park.

TEXAS PLAINS

The plains of Texas are broad expanses of country with very little surface relief. Most of the plains support grasses and some have wooded areas, particularly along stream valleys.

The plains of the northwestern part of the State have been subdivided as follows.

High Plains

This area (Pl. 9), often called “the caprock,” is an elevated plateau which rises above the rolling plains which surround it. The High Plains are bounded by the Pecos River valley on the south, southeast, and west and by the North-Central Plains on the east.

The surface of the High Plains is very flat and characterized by a sparse cover of grasses and few trees. The surface strata consist largely of unconsolidated deposits of sands and gravels of Quaternary and Tertiary age, with remnants of Lower Cretaceous limestones along the southern margin. The rocks of the High Plains are mostly unfossiliferous, but mammalian remains have been found at several localities.

HIGH PLAINS
NORTH-CENTRAL PLAINS
GRAND PRAIRIE
TRANS-PECOS TEXAS
VAN HORN UPLIFT
THE BIG BEND AREA
SOLITARIO UPLIFT
MARATHON UPLIFT
EDWARDS PLATEAU
LLANO UPLIFT
BALCONES FAULT ZONE
GULF COASTAL PLAIN

North-Central Plains

Surface strata of the North-Central Plains (Pl. 9) are westward-dipping Pennsylvanian, Permian, and Triassic rocks. Present also are extensive exposures of Quaternary sands and gravels which trend north-south across the central portion of the region. The area is bounded on the west by the High Plains, on the east by the Grand Prairie, and on the south by the Edwards Plateau and Llano uplift. Many vertebrate fossils have been collected from the Permian and Triassic rocks of this area. There are also many excellent outcrops of fossiliferous Pennsylvanian formations in the North-Central Plains region.

Edwards Plateau

The Edwards Plateau (Pl. 9) is located in south-central Texas and is bounded on the south by the Balcones fault zone and on the north by the North-Central Plains. The surface of the area is typically flat with a gentle slope to the south. The rocks of the Edwards Plateau consist primarily of Lower Cretaceous limestones and shales, many of which are very fossiliferous.

Grand Prairie

This area (Pl. 9) has a relatively flat surface but there are areas of gently rolling hills. The eastern boundary of the Grand Prairie is marked partly by the Balcones fault zone. North of McLennan County, however, the Balcones fault zone is not expressed at the surface and in this area the eastern boundary is defined by the western edge of the Woodbine exposures. Upper and Lower Cretaceous rocks occur at the surface and dip to the southeast; many of these rocks contain a large number of invertebrate fossils.

Llano Uplift

The Llano uplift (Pl. 9) is located in the central part of the State where Precambrian igneous and metamorphic rocks and sedimentary rocks of early Paleozoic age occur on the surface. The area, which now appears as a basin-shaped depression, was at one time covered by Lower Cretaceous rocks and perhaps also by Devonian, Mississippian, and Pennsylvanian strata. These have since been removed by erosion. The east, south, and west sides of the uplift are surrounded by Lower Cretaceous rocks, and the northern margin is marked by the Mississippian and Pennsylvanian formations of the North-Central Plains. The area is, in general, composed of unfossiliferous rocks, but some invertebrate fossils (primarily trilobites and brachiopods) have been collected.

GULF COASTAL PLAIN

The Gulf Coastal Plain (Pl. 9) is composed of Cretaceous, Tertiary, and Quaternary rocks and includes the eastern, southeastern, and southern portions of the State. The rocks of the area consist of sands, clays, shales, and limestones. The Texas Gulf Coastal Plain is bounded on the north and west by the Balcones fault zone, on the south and southwest by the Gulf of Mexico, and extends eastward into Arkansas and Louisiana.

The region has broad river valleys and uplands of low relief, but there is an increase in relief toward the interior of the State. The surface of the area slopes gradually toward the Gulf and successively younger formations are encountered gulfward.

The rocks of the Texas Gulf Coastal Plain are relatively unfossiliferous, but many of the Upper Cretaceous rocks contain fossils. In the central portion of the region some marine formations of Tertiary age locally contain well-preserved invertebrate fossils.

Geology

Geologic studies of the State of Texas have indicated the presence of rocks formed during every era and period of geologic time. These range from the Precambrian granites of the Llano uplift to the Quaternary gravels of the High Plains.

One of the best ways to become acquainted with the geology of Texas is to study the _geologic map_ of the State (Pl. 10). A geologic map shows the distribution and age of surface rocks and may also indicate what kind of geologic structures are present. The types of rocks that crop out at the surface may be shown by means of symbols, colors, or patterns, and these are explained by a legend which accompanies the map. On Plate 10, colors are used to show the distribution and geologic age of the surface rocks of Texas. Reference to this map will give the collector some idea of the age of the fossils that might be found in a given area. Some special geologic maps may have the location of geologic structures and formation contacts indicated by means of symbols, such as dashed lines, arrows, and similar special markings. However, the map included in this publication does not show any of these special markings.

PRECAMBRIAN ROCKS

The Precambrian rocks of Texas are composed of igneous and metamorphic rocks and some sedimentary rocks. Most of the Precambrian outcrops are in the Llano uplift and El Paso and Van Horn regions.

Alterations produced by vast amounts of time, heat, and pressure have obliterated any trace of fossils that may have been present in these rocks. With the exception of some questionable primitive plants collected in the Van Horn region, no Precambrian fossils have been reported from Texas.

PALEOZOIC ROCKS

Rocks of Paleozoic age are widespread in Texas, and rocks of each period are well exposed. Outcrops are found in the Llano uplift, North-Central Plains, and Trans-Pecos region. The most extensive exposures are of Pennsylvanian and Permian age, and the former are highly fossiliferous in parts of the North-Central Plains.

Cambrian

Rocks of late Cambrian age are exposed in the Llano, Marathon, and Solitario uplifts, and the Franklin Mountains near El Paso. These are sedimentary rocks consisting of conglomerates, sandstones, shales, limestones, and some dolomites.

Some of these formations are relatively fossiliferous, but the specimens are commonly fragmental and very poorly preserved. Fossils that are apt to be found in the Cambrian rocks of the Llano uplift include brachiopods, gastropods, trilobites, and small rounded objects believed to have been formed by algae (primitive one-celled plants). In other parts of the State, Cambrian rocks are sparsely fossiliferous and the fossils consist primarily of fragmental brachiopods, trilobites, and algae.

Ordovician

Ordovician outcrops are present in the Llano uplift of central Texas and in the Marathon, Solitario, El Paso, and Van Horn regions of Trans-Pecos Texas. These are sedimentary rocks and consist largely of sandstones, cherts, limestones, and dolomites.

Although some of the Ordovician formations are fossiliferous, they are seldom collected by amateur paleontologists because they are exposed in relatively inaccessible places and the fossils are usually poorly preserved. Ordovician fossils reported from Texas include sponges, corals, brachiopods, gastropods, cephalopods, and trilobites. In addition, the Marathon formation of the Marathon uplift contains large numbers of well-preserved graptolites (fig. 24, p. 86).

Silurian

The Silurian of Texas is poorly represented in surface exposures, and only one formation, the Fusselman, has been described. The Fusselman crops out in the El Paso and Van Horn regions where it is a white dolomitic limestone. Fossils are not abundant in this formation, but brachiopods and corals have been collected at a few localities.

Devonian

Devonian rocks are best developed in Trans-Pecos Texas, especially in the Marathon, El Paso, and Van Horn regions. In addition to the Trans-Pecos exposures, there are minor outcrops of Devonian rocks in the Llano uplift of central Texas.

Fossils are rare and fragmental in the Trans-Pecos exposures and consist primarily of radiolarians and brachiopods. The Devonian rocks of central Texas are predominantly calcareous and, although the material is usually poorly preserved, many fossils have been collected from them. These include bryozoans, corals, brachiopods, gastropods, and trilobites. Conodonts and fragments of primitive armored fishes (Pl. 37) have also been reported.

Mississippian

Mississippian rocks are exposed in the Llano region and in the Hueco Mountains of the Trans-Pecos area. The Trans-Pecos rocks primarily contain brachiopods with some bryozoans and gastropods.

The central Texas Mississippian rocks are much more fossiliferous and some of the material is well preserved. Fossils reported from this area include brachiopods (Pl. 17), crinoids, gastropods, cephalopods, trilobites, and ostracodes.

Pennsylvanian

Pennsylvanian rocks are well represented in Texas and are exposed in the Llano uplift, north-central Texas, and Trans-Pecos Texas.

In Trans-Pecos Texas fossiliferous rocks crop out in the Hueco and Diablo Mountains. Fossils found in this area are algae, fusulinids, corals, brachiopods, pelecypods, gastropods, cephalopods, and crinoids. There is also a thick section of Pennsylvanian rocks in the Marathon uplift, but only one formation, the Gaptank, is very fossiliferous. It contains many fossils including fusulinids, sponges, corals, bryozoans, brachiopods, gastropods, pelecypods, cephalopods, and crinoids.

Certain Pennsylvanian strata in the Llano region are very fossiliferous, and the material is well preserved. The more abundant forms are fusulinids, corals, brachiopods, gastropods, pelecypods, cephalopods, and crinoids.

Probably the best Pennsylvanian collecting areas are to be found in north-central Texas. Here the thick marine limestones and shales contain large numbers of well-preserved invertebrate fossils, and the terrestrial or shallow marine strata have yielded an abundance of plant fossils. Invertebrate fossils are apt to be found along the banks of streams and gullies and in railroad and highway cuts. Many of the limestones bear large numbers of fusulinids or crinoid stems, and the shales may contain many corals, brachiopods, and mollusks. The best collecting will, of course, be found where the rocks have been sufficiently weathered.

Typical invertebrate fossils are foraminifera (principally fusulinids), corals (especially the solitary or “horn” corals), brachiopods, bryozoans (the lacy and branching types are most common), pelecypods, gastropods (exhibiting a variety of coiling), cephalopods (nautiloids and goniatites predominate), and crinoids, which in many areas are found in thick crinoidal limestones (fig. 8). Some typical Pennsylvanian fossils are illustrated in Plates 14, 15, 17, 18, 19, 20, 21, 24, 32, and 35.

Permian

Permian rocks are found in widely separated areas in Texas. The best exposed section of marine Permian rocks is found in the Glass Mountains of Brewester County, and many of these rocks are very fossiliferous. The original shell material of some of the Permian fossils of this area has been replaced by siliceous material which is very well preserved. These silicified fossils are removed from the limestone by solution in acid, and some most remarkable specimens have been recovered in this manner (Pl. 3). Brachiopods are the most common fossils, but corals, bryozoans, and mollusks have also been recovered.

Extensive Permian exposures occur also in the central part of the North-Central Plains region. These rocks were formed from sediments of both marine and continental origin and some of them are fossiliferous. The marine rocks contain a variety of invertebrate fossils including brachiopods, pelecypods, gastropods, and ammonoids. Those rocks representing terrestrial deposits contain vertebrate remains at many localities, and numerous amphibians and primitive reptiles (Pl. 40) have been collected from them.

MESOZOIC ROCKS

Mesozoic rocks occur over a wide area of Texas and include exposures of Triassic, Jurassic, and Cretaceous age. Many of the Upper and Lower Cretaceous outcrops are quite fossiliferous and easily accessible and thus of considerable interest to many amateur collectors.

Triassic

Triassic rocks crop out in parts of the High Plains, the Glass Mountains of Trans-Pecos Texas, and parts of Pecos, Crockett, Upton, Reagan, and Glasscock and other west Texas counties. These are predominantly nonmarine rocks consisting of conglomerates, sandstones, shales, and some gypsum beds.

Triassic fossils are almost exclusively vertebrates, although some poorly preserved plant and invertebrate remains have been reported. Fossil vertebrates of the Texas Triassic include phytosaurs (Pl. 42), crocodiles, amphibians, and fish.

Jurassic

In Texas, surface exposures of Jurassic rocks are known only from Malone Mountain in southwestern Hudspeth County. The rocks there are limestones, shales, sandstones, and conglomerates. Fossils reported from that locality include marine and fresh-water pelecypods, fresh-water gastropods, and ammonites.

Cretaceous

Rocks of Cretaceous age are widely distributed in Texas and represent one of the more important rock systems of the State. Cretaceous outcrops occur in central Texas, north Texas, the Edwards Plateau, parts of the High Plains, the Gulf Coastal Plain, and Trans-Pecos Texas.

As mentioned earlier, the Texas Cretaceous has been divided into the Lower Cretaceous (Comanche series) and Upper Cretaceous (Gulf series). These rocks consist primarily of marls (a type of calcareous clay), shales, chalks, and limestones, but sands and conglomerates also occur. Cretaceous rocks occur on the surface of about 28 percent of Texas, and many of the larger cities of the State are situated on Cretaceous strata.

Many of the Gulf and Comanche formations contain fossils which are of interest both to amateur and professional paleontologists. Because of their wide distribution in and near large population centers, Cretaceous outcrops can be conveniently visited by many amateur fossil collectors. The fossils are usually abundant and varied, and some are well preserved. Although numerous kinds of fossils may be collected, the more common forms are cephalopods, pelecypods, gastropods, and echinoids. Some of the more typical Cretaceous fossils are shown in Plates 16, 21, 25-28, 32, 33, 35, and 36.

Cretaceous fossils are more commonly found in shales and chalky limestones. Fossiliferous outcrops of these rocks can be found along many streams, roads, and highways of central Texas, north Texas, and the Edwards Plateau. Outcrops which have been weathered are more likely to provide good collecting. In general, collecting is poor in areas covered with heavy vegetation or recent stream deposits. Good collecting localities are outcrops which have a fairly steep slope with a covering of weathered rock material and a minimum of vegetation. One should move slowly from the base of the slope upward while searching the ground for any evidence of fossils, and particular attention should be given to any small gullies since these often contain fossils that have been washed out of upper beds in the exposure.

CENOZOIC ROCKS

Cenozoic rocks are widespread in Texas but occur primarily in a broad belt along the Gulf Coastal Plain. In addition, there are exposures of nonmarine Cenozoic strata in the High Plains, North-Central Plains, and Trans-Pecos region. There are also many exposures of Cenozoic igneous rocks in Trans-Pecos Texas.

Rocks of Cenozoic age occur in more than one-third of Texas and consist of conglomerates, sands, clays, and some limestone and lignite beds.

Tertiary

Extensive exposures of Tertiary rocks trend northeast-southwest in a broad band across the Gulf Coastal Plain area. These strata, consisting of sands, clays, and poorly consolidated limestones, are underlain by Cretaceous rocks.

Invertebrate fossils are common in certain Tertiary formations and pelecypods, gastropods, and corals are the predominant forms. In general, however, fossiliferous exposures are of local occurrence and most of the Tertiary formations are unfossiliferous. Those Tertiary invertebrates that are present, however, are often well preserved and represent many interesting types (Pls. 16, 22, 23, 29, 30, 31).

Tertiary invertebrate fossils are commonly found in sands, clays, and marls. Many of these sands and marls have a green color which is due to the presence of glauconite (a green mineral containing iron and closely related to the micas). At certain localities on the Gulf Coastal Plain the glauconite marls and sands of the Weches and Crockett formations contain large numbers of well-preserved clams, snails, and corals. Fossiliferous exposures of Tertiary rocks are sometimes found in road cuts, but better exposures may be found along the banks of rivers and creeks. Certain bluffs along the Brazos, Sabine, and Trinity rivers are well-known Tertiary fossil collecting localities. Many of these better localities are listed in some of the Bureau of Economic Geology bulletins included in the bibliography of this publication (pp. 109-110).

Quaternary

Quaternary deposits of Pleistocene age (geologic time scale, Pl. 1) are found in many parts of Texas and consist of sands, clays, and gravels.

These rocks are distributed along the Gulf Coast in a belt from 50 to 100 miles wide. They occur also as stream terraces in the Edwards Plateau and North-Central Plains regions. In addition, Quaternary sands and gravels are widely distributed over the surface of much of Trans-Pecos Texas. There are also fossiliferous Pleistocene strata in the High Plains region.

Invertebrate fossils are rare in Pleistocene rocks, but some fresh-water and terrestrial mollusks occur. Vertebrate remains, however, are abundant in many localities, and large numbers of horses, camels, mammoths, and other mammals (Pls. 46-49) have been collected. Fossil bones and teeth (figs. 25, 26, p. 104) are commonly found in the gravels and sands of many of the river terraces of the State.

MAIN TYPES OF FOSSILS

The beginning fossil collector is usually amazed by the many different plants and animals that have left some trace of their existence. In order to understand these different types of prehistoric life, it is necessary to know something about the organisms that are living today.

This handbook discusses the more important groups of plants and animals which have left some sort of paleontological record, and each major group begins with a discussion of the more simple organisms and continues through the more advanced forms. Because scientific workers do not always agree on exactly the same classification, the system adopted in this handbook contains the latest ideas of several workers. It is simple enough to understand, yet complete enough to help one know and classify his fossils. It should be noted that this classification may differ in some respects from that of certain older paleontological publications. Therefore, it has seemed advisable to list other names for some of the groups that are discussed.

In some instances, the brief descriptions and illustrations of each group will enable the collector to make a preliminary identification of his fossils. For more detailed information about each group, the reader should refer to “Books About Fossils” (pp. 108-110).

This part of the handbook begins with a brief summary of the major groups of the plant kingdom, followed by a discussion of the characteristics and relative paleontological importance of the various invertebrate animals. Emphasis is placed on the invertebrates because this type of fossil is most commonly collected by the amateur. Finally, there is a general review of the vertebrates.

PLANT FOSSILS

Plant fossils are usually fragmental and poorly preserved, and this tends to discourage most amateurs from an active interest in paleobotany. However, in spite of these problems, much is known of the evolution of plants, and plant fossils provide much information about life of the past. In addition, certain plants are of considerable value as indicators of ancient climatic conditions, and their remains have played a large part in the formation of vast coal deposits.

Classification of the Plant Kingdom

In the following classification only the larger taxonomic groups are discussed. Notice that the term _division_ has been used in place of the term phylum as used in the animal kingdom. This usage is now preferred by many botanists and paleobotanists.

DIVISION THALLOPHYTA

Thallophytes are simple plants without roots, stems, or leaves. They include the fungi, algae, and diatoms (Pl. 12). Diatoms are microscopic fossils that are found in many of the rocks of Texas, and they are quite abundant in Recent sediments as well. Certain of the Paleozoic limestones of central Texas contain banded spherical masses of algae called “algal biscuits.” Although not particularly useful fossils, thallophytes have a long geologic history and are known in rocks ranging from Precambrian to Recent in age.

DIVISION BRYOPHYTA

The bryophytes are simple rootless plants and include the mosses and liverworts. Although more complex, the bryophytes resemble the algae in some respects. They are uncommon fossils, but undoubted bryophytes (liverworts) have been reported from rocks as old as Mississippian.

DIVISION TRACHEOPHYTA

This division has been divided into four subdivisions, among which are many of the more common living and fossil plants. Such important plants as the ferns, evergreens, hardwood trees, and the flowering plants are all tracheophytes. Among the more common and abundant fossil tracheophytes are the ferns, cycads, and _Gingko_, in addition to such important “coal plants” as the scale trees, club mosses, and scouring rushes (Pls. 12, 13). The latter commonly occur in many of the world’s great coal deposits, and their remains make up a large part of the coal. Plant fossils of this type may be collected in the dumps around some of the abandoned coal mines in north-central Texas and from other Pennsylvanian rocks in north and Trans-Pecos Texas.

PRECAMBRIAN
PALEOZOIC
CAMBRIAN
ORDOVICIAN
SILURIAN
DEVONIAN
MISSISSIPPIAN
PENNSYLVANIAN
PERMIAN
MESOZOIC
JURASSIC
CRETACEOUS
CENOZOIC

Thallophyta
Bryophyta
Tracheophyta
Protozoa
Porifera
Coelenterata
Bryozoa
Brachiopada
Mollusca
Annelida
Arthropoda
Echinodermata
Chordata

DIATOMS × 900
ALGAE × 400

LEPIDODENDRON × ½
SIGILLARIA × ½

NEUROPTERIS × ½
PSILOPHYTON × ⅓
CALAMITES × ½
AMELANCHIER × ½
CORDAITES × ¼
GINGKO × ½

Fairly well-preserved plant remains may also be collected from the Woodbine group of the Upper Cretaceous in north Texas, and fossil wood, most of it silicified, has been reported from rocks of almost all ages and in almost every section of the State. In addition, some of the carbonaceous clays and shales of east Texas contain large assemblages of plant leaves, which in some places are well preserved.

It is also possible to find the fossilized remains of seeds, spores, and pollen. Because of their small size, these minute remains are not destroyed by the drill bit and can be brought out of deep wells without being damaged, and for this reason they are a valuable tool for the micropaleontologist.

ANIMAL FOSSILS

The fossilized remains of animals are very common in many of the sedimentary rocks of Texas. These remains are of many different kinds and represent the fossils of such diverse organisms as the shell of a tiny one-celled animal or the bones or tusk of a huge elephant. The fossils most commonly found, however, are the remains of invertebrate animals such as clams, snails, and corals, and it is this type of fossil that attracts the interest of most amateur collectors.

It is not always easy to tell whether certain organisms are plants or animals, and because of this some scientists have suggested that these “in-betweens” be placed in a separate kingdom—the Protista. The protistans are primarily unicellular organisms and are represented by such forms as bacteria, algae, diatoms, and the protozoans (see below). But in this publication, only the plant and animal kingdoms are recognized.

Phylum Protozoa

This phylum is composed of simple one-celled animals many of which have no shell or external body covering. Some, however, have external hard parts that can become fossilized, and these forms are quite useful microfossils.

CLASS SARCODINA.—

This class contains a group of one-celled animals which may secrete an exoskeleton (external protective covering) of chitin, silica, or calcium carbonate. Included in this class are foraminiferans (commonly called forams) and radiolarians.

Order Foraminifera.—

Members of this order secrete tiny chambered shells which are very useful microfossils. The forams are predominantly marine organisms and have shells composed of chitin, silica, or calcium carbonate. In addition, some forms construct a shell of sand grains or some other material which is cemented together by a sticky substance that is secreted by the animal.

Forams are very abundant in the rocks of Texas and particularly so in rocks of Mesozoic and Cenozoic age. The most numerous and easily observed Paleozoic foraminiferans are the fusulinids (fig. 9a), and their small spindle-shaped remains are very abundant in many of the Pennsylvanian limestones of north-central and Trans-Pecos Texas. Some typical Texas forams are illustrated in figure 9.

Order Radiolaria.—

The radiolarians (fig. 10) have delicate spine-covered shells composed of silica, and their remains are very abundant in certain recent marine sediments. They may also be found as fossils and have been reported from Devonian and Permian rocks in Trans-Pecos Texas, and probable radiolarians have been reported from still younger beds.

Phylum Porifera

These are sponges and are the simplest of the many-celled animals. Living sponges secrete a skeleton which may be composed of chitin, silica, or calcium carbonate. These substances are commonly found in the form of spicules—tiny hard parts that are used to help support the soft tissues of the animal. These spicules take on a variety of shapes (Pl. 14) and are occasionally found as microfossils in some marine sediments.

Although sponges are not particularly common fossils, their remains occur in some parts of the State. Sponges have been collected from Paleozoic and Mesozoic formations of north and Trans-Pecos Texas, and their spicules have been reported from well cuttings.

Phylum Coelenterata

The coelenterates are multicelled animals which, though more complex than the sponges, are rather primitive animals. The living animal is characterized by a sac-like body cavity, a definite mouth, and tentacles which bear stinging cells. Some forms, for example, the jellyfishes, have an umbrella-shaped body and are single free-moving organisms. Others, like the colonial corals, are composed of many individuals living together in a colony.

Most zoologists and paleontologists recognize three classes of coelenterates: (1) the Hydrozoa, containing the small animals known as hydroids, (2) the Scyphozoa, which includes the jellyfish, and (3) the Anthozoa, which includes the corals and sea anemones. Because of their extreme fragility and lack of hard parts, hydrozoans and scyphozoans are not commonly found as fossils. They do, however, have a long geologic history and may be preserved when unusual conditions of fossilization occur. The anthozoans, especially the corals, are by far the most important class geologically, and these forms have left a very good paleontological record.

CLASS ANTHOZOA.—

This class is composed of a group of exclusively marine organisms and includes the corals and sea anemones. The coral animal, or _polyp_, secretes a cup-shaped calcareous (limy) exoskeleton. This skeleton, called a _corallite_, is usually divided by radial partitions called _septa_. The polyp lives in the _calyx_, which is the central bowl-shaped depression in the top of the corallite (fig. 11a).

_Solitary_ corals form an individual corallite for each polyp, and because of their shape these may be given such names as “horn corals” (_Lophophyllidium_, Pl. 15) or “button corals” (_Micrabacia_, Pl. 16). _Colonial_ or _compound_ corals (Pl. 15) live together in colonies, which are formed of many individual skeletons attached to each other (fig. 11b), and the compound mass of coral skeletons formed in this manner is called a _corallum_. Fossil corals commonly occur in many marine limestones and in places constitute a large portion of the rock.

a
Columella
Septum
Corallite
b
Calyx
Septum
Corallum

The class Anthozoa has been divided into several subclasses, but only one, the Zoantharia, is of paleontological importance.

Subclass Zoantharia.—

Most corals and all sea anemones belong to this subclass. Zoantharians are either colonial or solitary and, because most of them possess a hard preservable exoskeleton, they are the most important group of anthozoans geologically. The various orders of the subclass Zoantharia are discussed below.

Order Rugosa.—

These are corals in which the septa are arranged in cycles of four. Both solitary and colonial forms occur, and they are found only in rocks of Paleozoic age. Rugose corals are abundant in many of the Paleozoic formations of Texas, and two of the more typical forms (_Lophophyllidium_ and _Caninia_) are illustrated in Plate 15. Members of this order have been placed in the subclass Tetracoralla of older classifications.

Order Scleractinia.—

The scleractinians are solitary or colonial corals in which the septa grow in multiples of six, and they are the most important and abundant of the modern corals. These corals were the dominant reef builders of Mesozoic and Cenozoic seas, and their remains are common in many of the marine formations of the State. Plate 16 illustrates some typical scleractinian corals from the rocks of Texas. This order has also been referred to as subclass Hexacoralla, and its members have been called hexacorals.

Order Tabulata.—

These are corals that are now extinct but are known from fossils in both Paleozoic and Mesozoic rocks. Tabulate corals are characterized by horizontal partitions called _tabulae_, and septa are absent or poorly developed. The tabulates were the most abundant reef-building corals during Paleozoic time and are well known as fossils. Because of certain similarities with other anthozoans, some paleontologists have treated the Tabulata as a distinct subclass rather than as an order of the Zoantharia.

Tabulate corals are not uncommon in many of the Paleozoic rocks of Texas, and two of these (_Cladochonus_ and _Striatopora_) are illustrated in Plate 15.

Phylum Bryozoa

Bryozoans are colonial animals that are often referred to as “sea mats.” They have been called this because they are commonly found matted on shells, rocks, fossils, and other objects. The living animal is quite small, has a tentacle-bearing ridge surrounding the mouth, and secretes a tiny cup-like exoskeleton composed of calcareous or chitinous material. These little chambers, known as _zooecia_ (or _autopores_), are seen as small pits on the surface of the bryozoan colony (_Rhombopora_, Pl. 17). The zooecia grow together to form the bryozoan colony, and some fossil colonies grow to be as much as 2 feet across. Such colonies may be spiral (fig. 12b), branching, or lace-like (fig. 12a), and the latter two types are very common in many of the fossiliferous strata of Texas. Undoubted bryozoan fossils have been recorded in rocks of Lower Ordovician age, but questionable Cambrian forms have also been reported. Bryozoans are abundant in the seas of today, but only a few forms inhabit fresh waters.

MEANDROSTIA × 1
HELIOSPONGIA × 1
ASTRAEOSPONGIUM × ½
ASTYLOSPONGIA × ½
RECEPTACULITES × ½
GIRTYOCOELIA × 2