Chapter XIII (1)

FOSSIL ARTICULATA; COMPRISING THE ANNELIDES, CIRRIPEDES, CRUSTACEANS, AND INSECTS.

The division of the Animal Kingdom termed _Articulata_, embraces, as the name implies, those animals which have a _jointed_ body, generally possessing an external-jointed skeleton, composed of segments more or less annular and distinct. It comprehends six classes; namely--

1. Annelata, or Annelida; _i. e._ formed of rings; comprising the Red-blooded Worms.

2. Myriapoda; as the Centipede.

3. Cirripedia; _i. e._ having curled-feet; as the _Balanus_ and _Lepas_ (Barnacle).

4. Crustacea; as the Crab, Lobster, and Water Flea.

5. Arachnida; Mites, Scorpions, and Spiders.

6. Insecta, or Insects.

Of the first, third, fourth, and sixth of these classes, remains occur in the British strata, some being referable to existing, but the greater part to extinct species and genera. I propose to describe a few illustrative examples of the fossils belonging to each Class.

Annelida.--This name is given to a class of _Articulata_, consisting of worms, whose bodies are formed of little rings, or annular segments, and which have red blood; as the Leech, Earth-worm, &c. Some are naked (the _Dorsibranchiata_ and _Abranchiata_), and move with great celerity; as the _Gordius_, or Hair-worm, and the _Nereis_, so frequent on the sands of the sea-shore. Others have shelly coverings (the _Tubicolæ_), as the _Serpula_, and are sedentary, or fixed to other bodies. The soft bodies of certain species are protected by a coat, or tube, formed by the agglutination of sand, or other foreign substances, as in the _Sabella_ (_Lign. 123, fig. 6_, p. 385).

The fossil remains of the testaceous Annelides are very abundant in some deposits; and even the naked, flexible, soft-bodied forms have left proofs of their existence in some of the most ancient sedimentary rocks. Traces of nine species, belonging to five genera of these soft, naked Annelides, have been observed in the Silurian strata of Britain.

(_Drawn by Miss Murray._)

Nereites Cambrensis. (_Murch. Sil. Syst._) _Llampeter._]

[Sidenote: FOSSIL NEREIS. SERPULA. CIRRIPEDIA.]

The first notice of these remarkable remains appeared in the invaluable work of Sir R. I. Murchison on the Silurian System.[430] The living species of Nereis (_Dorsibranchiate_) are free, agile animals, having a distinct head, provided with either eyes or antennæ, or both; they are the most perfect in structure of all the Annelides. The fossil represented in _Lign. 166_ indicates that the body of the original was composed of about one hundred and twenty segments; the feet were half the length of a segment of the body; and the _cirri_ of the feet were longer than such segment. A more slender species, (_Nereites Sedgwickii_,) the body consisting of a greater number of segments, is also figured and described by Sir It. I. Murchison. Other impressions in the same stone resemble those that would be produced by smooth Annelides (_Abranchiate_) related to the _Gordius_, or Hair-worm.[431]

[430] Murch. Sil. Syst. p. 699.

[431] Murch. Sil. Syst. p. 701, pl. xxvii.; and M’Coy, Cambridge Pal. Foss. p. 128, pl. 1. D.

Serpula.--The animals of this genus are sedentary or fixed, having calcareous tubes or shells, but to which they have no muscular attachment. They have plumose or arborescent gills affixed to the anterior part of the body. The shelly tubes of the _Serpulæ_ are constantly seen on our coasts, encrusting stones, rocks, shells, sea-weeds, &c., and may be known by their contorted or twisted forms. There are a hundred and fifty British fossil Tubicolæ. A large species has been discovered in the Silurian rocks (_Murch. Sil. Syst._ pl. v. _fig._ 1); several occur in the Carboniferous, Oolitic, and Cretaceous, and many in the Tertiary strata. In the Upper Chalk, a smooth tortuous _Serpula_ is not uncommon (_S. plexus_, _Min. Conch_, tab. 598); it occurs in masses several inches long. But I have not observed either in the Chalk, or in any other deposit, indications of banks of _Serpulidæ_, like those now in progress off the Bermudas, and which resemble coral-reefs in their solidity and extent.

Cirripedia.--These animals have a soft body, enveloped in a membrane, which in some genera is protected only by a horny sheath, but in general is enclosed in a shell composed of various calcareous plates.[432] They have six pairs of feet, terminating in long, slender, articulated tentacula, furnished with cilia, and coiled up like tendrils at the extremities near the mouth. The name of the class (_curled-feet_) has originated from the appearance presented by the curled tentacula when projecting from the oval aperture of the shell. The testaceous Cirripedes or _Barnacles_ are divided into two groups; namely, the _sessile_, or those which in their adult state are fixed by the base to other bodies, (Balanidæ,) as the Acorn-shell, or _Balanus_; and the _pedunculated_, which have a process of attachment, peduncle or stem, (Lepadidæ,) as the Duck-barnacle, or _Lepas_. But the young animals of these genera have powerful locomotive organs, and are capable of swimming, by sudden jerks, like some of the crustaceans, to which class, especially in this stage of their existence, they closely approach. But after a short period of freedom, the young Cirripede fixes itself in some locality suitable to its economy, and rapidly undergoes the transformation which results in the sessile adult Barnacle or Lepas.[433]

[432] Until within the last few years the valves of the _Cirripedes_ have been generally figured and described in works on Conchology as being allied to the _Mollusca_.

[433] See Mr. C. Darwin’s admirable Monograph on the Cirripedia, published by the Ray Society, 1851.

Balanus. _Lign. 167, fig. 1.3_--The shell of this sessile Cirripede is of a conical shape and cellular structure. It consists of a thick plate at the base, or place of attachment; of a series of plates, united by sutures, arranged around the body of the animal, and called _parietal_ valves; and of pieces termed _opercular_ valves, by which the aperture is closed. The shell of the _Balanus_, so common on the rocks of our shores, and on every pile and pier within reach of the tide, is composed of six parietal, and four opercular valves. The fossil _Balanus_, _Lign. 167. fig. 1_, is from the Crag, a formation containing many shells of this genus: in this example, the six parietal plates of the conical shell only remain, but in some specimens the opercular valves are also preserved. There are about twelve species of Balanus in the Crag; but none have been found either in the older Tertiary, or in the secondary rocks of England. In the newer _Pliocene_ deposits of the Sub-Apennines, and of North America, several species are common.

Fig. 1.--Balanus tesselatus. (_Sow. Min. Conch._) _Crag._ _Suffolk._
2.--Loricula pulchella. (_Mr. G. B. Sowerby, jun._) _Chalk._ _Kent._
_a._ The situation of the aperture of the shell.
3.--A valve (_Tergum_) of Scalpellum maximum. _Chalk._ _Lewes._
4.--Another valve (_Carina_) of the same species.
5.--Pholas priscus[434]; in wood. _Lower Green Sand._ _Sandgate._
_a._ One of the shells seen in profile.
6.--Univalve Shells (_Neritæ_) in hollows formed by _Pholades_.
(_Mr. Bensted._) _Lower Green Sand._ _Maidstone._
_a._ The _Nerites_ lodged in the cavities.
_b._ Fragment of wood remaining attached.
]

[434] Figs. 5 and 6 are referred to and described at pages 409 and 410, vol. i.

Lepadidæ.--The pedunculated Cirripedes, of which the common _Duck-barnacle_ is a well-known example, have a strong, muscular, hollow stalk, or peduncle, which supports a multivalve shell, containing the body of the animal. In _Pollicipes_ and _Scalpellum_ there are small calcareous plates covering the junction of the body with the peduncle. Detached valves of several species of these two genera are met with in the Chalk, Galt, and Shanklin Sand, of Kent and Sussex; and three species have been found in the English Tertiary beds.[435] The valves of these fossil Cirripedes are most usually found in a disconnected state (_Lign._ 167, _figs._ 3 and 4); but the Tertiary _Scalpellum quadratum_ is sometimes better preserved; and Mr. Morris has described a most beautiful group of _Pollicipes_ (_P. concinnus_), attached to an Ammonite, from the Oxford Clay. Mr Wetherell, of Highgate, has also discovered, in the Upper Chalk, near Rochester, an almost perfect Cirripede, named by Mr. G. B. Sowerby, jun., _Loricula pulchella_ (_Lign. 167, fig. 2_), and lately more fully described in detail by Mr. Darwin.[436]

[435] See Mr. Darwin’s Monograph of the Fossil _Lepadidæ_, Palæontographical Society, 1851.

[436] Monograph of the Fossil Lepadidæ, p. 81, pl. v.

Crustacea.--The animals whose fossil remains we have now to consider, are characterised by their crustaceous external integument or shell, which is disposed in segments, more or less distinct, the annular portions supporting articulated limbs or appendages. They are aquatic, free, locomotive beings, and possess distinct _branchiæ_, or organs fitted for aquatic respiration. The Crab and Lobster are examples of those tribes in which the external crust is calcareous, and "coloured by a pigmental substance, diffused more or less irregularly through it; and is formed upon and by a vascular organized integument, or corium, which is lined by the smooth serous membrane of the visceral cavities." (_Owen._) The subdivisions of this class have relation to the forms, combinations, and proportions of the primary rings or segments of the external crust or integument, but it will not be requisite for our present purpose to enter upon this department of the subject. It may, however, be necessary to mention, that in the normal type of Crustaceans, the integument consists of twenty-one rings or segments, which form the three regions into which the body is divided; namely, the head or _cephalic_, the _thoracic_, and the _abdominal_; each of which is assumed to consist of seven rings, although some of these are generally anchylosed, and form but one segment; and even the three regions are occasionally more or less blended together. The cephalic portion of the crust contains the principal organs of sense, and the commencement of the digestive apparatus, and includes the masticatory appendages. The _thoracic_ portion is formed of the rings to which the extremities serving for locomotion are attached: and, together with the cephalic, contains almost all the viscera. The consolidation of the rings or segments takes place most generally in the cephalic, and next in frequency in the thoracic; and but rarely occurs in the abdominal region. These animals possess organs of sight variously modified, and in some species highly complicated; some have smooth or simple eyes (_stemmata_), and others compound eyes, like those of insects, with distinct facets. In one grand division (called _Edriopthalmia_), the eyes are sessile and immovable; in the other (_Podopthalmia_), they are supported upon moveable stems or peduncles. These few remarks on the organization of the recent crustaceans are required, to make our description of the fossil remains intelligible to the general reader. As the shell, or calcareous integument, even in those species in which it is very dense and thick, is moulded upon the soft parts it envelopes, the experienced naturalist is able, from its configuration alone, to obtain certain conclusions as to the form, size, and position of the contained viscera; and, as these animals annually shed their solid case and acquire a new one, which is moulded on the soft parts, the form and relative situation of the internal organs must necessarily be faithfully represented by the external integument, even when it has acquired its greatest degree of consolidation; thus the regions of the stomach, heart, branchiæ or respiratory organs, &c. may be distinctly traced on the external shell. Hence the fossil carapaces may afford important data regarding the structure and economy of the extinct species. M. Desmarest[437] was the first naturalist who successfully applied this _phrenological method_ to the investigation of the fossil crustaceans.

[437] Histoire Naturelle des Crustacés Fossiles; par MM. Alex. Brongniart et Desmarest. 1 tom. 4to. Paris, 1822.

The fossil remains of Crustaceans consist of the calcareous covering or carapace, with the articulated extremities, and, rarely, the jaws and antennæ. For the most part, the specimens are mutilated, and present only portions of the carapace, abdominal segments, and detached claws; but in strata composed of very fine detritus, such as the cream-coloured limestones of Solenhofen and Pappenheim, examples often occur in the most beautiful state of preservation, appearing as if the animals had been carefully embalmed in a soft paste, that had quickly consolidated around them, and preserved them without mutilation or blemish. In some examples, even the colour of the original remains. The specimens found in hard limestones and coarse conglomerates are generally mutilated, and, as the under surface of the carapace, and the sternal plates to which the legs are attached, present more irregularity than the dorsal portion of the shell, they are firmly impacted in the stone, so as to render the development of some of the most important characters difficult, if not impossible. The antennæ and claws are often separated, or altogether wanting; the most common relics being the pincers and the carapace, or united cephalo-thoracic segments. The substance of the shell, which in the recent state consists of phosphate and carbonate of lime, with gelatine or cartilage, is commonly a friable carbonate of lime, tinged with oxide of iron. These remarks apply more particularly to the crabs, lobsters, shrimps, prawns, &c. Numerous species of the smaller crustaceans, as the_ Cypris_, and the extinct family of _Trilobites_, occur in myriads, and, in some formations, are the principal constituent of deposits of great thickness and extent.

The remains of this class have been found throughout the vast series of the fossiliferous strata. Extinct forms appear in prodigious numbers in the most ancient formations, and are succeeded by genera which approach more nearly to the more highly organized crustaceans. The Crab and Lobster tribes are represented by certain species in the Lias, Oolite, and Chalk; while in many of the Tertiary strata the existing types prevail.

The London Clay, in the Isle of Sheppey, yields many beautiful examples of the higher order of crustaceans, as the Crab, Lobster, &c. In the Chalk these remains are more rare, but a few fine specimens have been obtained. On the Continent, certain localities are extremely rich in these remains. Upwards of sixty species were discovered by Count Münster in the Jura limestone, at Solenhofen; and the Muschelkalk of Germany has yielded several extinct genera. The beautiful state in which these fossils occur, is exemplified in the specimen from Solenhofen, figured in the frontispiece of this work; _Pl. I. fig. 2_.

Fossil Crabs.--Of the _brachyurous_, or short-tailed, crustaceans, of which the common Crab is an example, and of the _Anomura_ (abnormal-tailed), there are remains of several genera in the Tertiary deposits. The Isle of Sheppey is the most productive locality in England. The carapaces of several kinds occur in the septaria and nodules of indurated clay; the chelate hand-claws (_pincer-claws_) are often found detached, and sometimes in connexion with the shield. The most numerous specimens are referable to two species. One of them (_Cancer Leachii_) is from two to three inches wide, and has a convex shell, the surface of which is covered by minute punctations, with three tubercles on each anterior lateral margin.[438] The carapace of the other species is more distinctly lobed, and studded with aculeated or spiny tubercles; it is named _C. tuberculatus_.[439]

[438] Hist. Nat. Crust. Foss. pl. viii. figs. 5 and 6.

[439] König, Icones Foss. Sect. _fig._ 54. These two species are now referred to the genus _Zanthopsis_.

A species of Crab, characterised by its relatively large claws, is common in the soft Tertiary limestone of Malta; and examples, in a fine state of preservation, are often seen in cabinets; see _Wond._ p. 251.

I am not aware that vestiges of more than one genus of brachyurous crustaceans have been observed in the British Secondary formations; namely, the _Podopilumnus Fittoni_ (M‘Coy), from the Greensand of Lyme Regis: but several small species belonging to the anomurous group have been found in the Galt.

In the _Galt_ at Ringmer, a village near Lewes, I discovered, many years since, four or five species of small crustaceans, which are figured and described, _Foss. South D._ pl. xxiv.; their natural relations were pointed out to me by the late eminent naturalist, Dr. Leach. Specimens of two of the species have since been collected at Folkstone, but as these are only the carapaces, no additional light has been thrown upon the structure of the originals.

The smallest species consists of the carapace or cephalo-thoracic segments, united into a transversely obovate, obscurely trilobate shell, the surface of which is covered with minute irregular papillæ; with four tubercles on each lateral portion, and an irregularly tuberculated dorsal ridge (see _Lign. 168, fig. 1, 1a._); it probably belongs to the genus Etyus; and I have named it _Etyus Martini_, in honour of my friend, P. J. Martin, Esq., author of several excellent Memoirs on the Geology of Western Sussex.

Fig. 1. --Etyus Martini (G. A. M.): showing the empty carapace
or shell. _Ringmer._
1_a_.--The dorsal surface of the same.
2. --Corystes[A] Stokesii (G. A. M.). _Ringmer._
3. --Corystes[440] Broderipii (G. A. M.). _Ringmer._
3_a_.--The under surface of the same species, displaying the
sternal plates and the bases of the claws, three of which
are marked _a, a, a_.
]

[440] These two crustaceans are included in the fossil genus Notopocorystes, established by Professor M’Coy, 1849.

There are two species that appear to have some relations with _Corystes_, a genus which includes several recent crustaceans that inhabit our shores, and are characterised by their elongated oval shell and four antennæ, the external pair being long, setaceous, and furnished with two rows of cilia. The tail is folded under the body when the animal is in repose. They have ten legs, the anterior pair _chelate_ (_with pincers_), the others terminating in an acute elongated nail or claw. The fossils consist of the carapace, and one example possesses the inferior or thoracic plates and the remains of the bases of some of the legs (see _Lign. 168, fig. 3a_).

Notopocorystes Stokesii. _Lign. 168, fig. 2._--The carapace is relatively wider than in most species of this genus; is has a strong dorsal ridge of irregular oblong tubercles; the union of the cephalic and thoracic segments is marked by a transverse undulated groove; there are three or four tubercles on the surface of each lateral portion of the former, and one on each of the latter. The whole surface is finely granulated. The openings left by the attachment of the peduncles of the eye remain.

Notopocorystes Broderipii. _Lign. 168, fig. 3._--This species, like the former, has a transverse undulated furrow, indicating the union of the cephalic with the thoracic segments; the dorsal ridge is smooth, and there are two tubercles on each lateral cephalic portion of the shield. The carapace is longitudinally ovate, much depressed, with three sharp points directed forwards on each margin of the anterior part: the whole surface is finely granulated. In the specimen _fig._ 3_a_ the sternal plates, with portions of the first joints of the claws, remain; one example (figured _Geol. S. E._ p. 169) possessed six or seven arcuate abdominal segments, which were turned under the body.[441]

[441] I have described these small crustaceans somewhat minutely, and have given them specific names, in the hope of directing the attention of collectors to these highly interesting relics, and leading to the discovery of more illustrative examples. See Foss. South D. pp. 96, 97.

The carapace or shell of the other crustacean observed in the Sussex Galt (_Notopocorystes Bechei_) is of an orbicular inflated form (see _Geol. S. E._ p. 169, _fig._ 3), and ornamented with twelve or thirteen aculeated tubercles; its margin is dentated.

In the friable arenaceous limestone of the Cretaceous formation at St. Peter’s Mountain, near Maestricht, the cheliferous claws of a small kind of crustacean (_Mesostylus Faujasii_, _Wond._ p. 338), are frequently discovered (and occasionally in the Chalk of Kent and Sussex), but with no vestige of the carapace or shell. This curious fact is explained by the analogy existing between the fossil claws and those of the _Pagurus_, or Hermit-crab, whose body is only covered by a delicate membrane, the claws alone having a calcareous covering; hence the latter might be preserved in a fossil state, while no traces of the soft parts remained. In the fossil, as in the recent claws, the right arm is the strongest. There is no doubt that the crustaceans to which the fossil claws belonged possessed the same modification of structure as the recent (anomurous) Hermit-crab, and must have sought shelter in the shells of the mollusks with which their durable remains are associated.

[Sidenote: FOSSIL LOBSTERS.]

Fossil Lobsters.--The _macrurous_, or long-tailed, crustaceans, as the _Lobster_, are distinguished from those of the former divisions by the prolonged abdomen (or tail, as it is commonly termed), which forms a powerful instrument of locomotion, and enables the animal to dart backwards through the water with great rapidity; and this is furnished with an appendage or tail, which none of the ambulatory crustaceans possess.

Of the fresh-water species, the Cray-fish (_Astacus fluviatilis_), and of the marine, the Lobster (_Astacus marinus_), are illustrative examples. The remains of three macrurous species occur in the London Clay of the Isle of Sheppey, associated with congenerous crustaceans; and the segments of the tails (_post-abdomen_) are often well preserved.

The Chalk contains a few species of the _Macrura_, which were first discovered in the quarries near Lewes, and are figured in _Foss. South D._ tab. xxx. xxxi.; they are among the most rare and delicate of the fossils of the Cretaceous strata. These remains consist of the carapace and claws, and rarely of the tail and antennæ, and are composed of a friable earthy crust, which, when first discovered, is of a dark chocolate colour, but quickly changes to a pale fawn, or reddish brown, by exposure to the air. In the specimens obtained by breaking the stone, the crustaceous covering remains attached by its rough external surface to one portion of chalk, and on the corresponding piece are sharp casts of the carapace and claws, having a glossy surface covered with minute papillæ formed by the bases of tubercles or spines. Four species have been observed.

Enoploclytia Leachii. _Lign. 169, figs. 1, 2, 3._--This is a long delicate crustacean, having a pair of equal, slender, anterior chelate claws, the fingers of which are long, attenuated, and armed with a row of obtuse cylindrical spines. The surface of the hand-claws is muricated, or covered with short erect aculeated tubercles. The pincers in the specimen figured _Lign. 169, fig. 3_, are shorter than in most examples. The carapace is elongated and sub-cylindrical, with a dorsal ridge and two lateral furrows, indicating the normal division of the cephalic and thoracic segments of the shield; the antennæ are long, filiform, and setaceous (_bristly_), and are placed on squamous peduncles (see _Lign. 169, fig. 2_).

There appear to have been five legs on each side; the anterior or chelate pair are the most usual relics of this animal; of the other claws and the branchiæ, but obscure indications have been obtained. The abdominal segments are arcuate, and six or seven in number; their surface is granulated; the appendage, or tail, is foliaceous and marginate, with a few longitudinal ridges (see _Lign. 169, fig. 1_).[442] The claws of these crustaceans may be easily recognised by their general aspect, and the length and straightness of the fingers or pincers. In most examples traces remain of the tendinous expansion of the muscles of the moveable claw (see _Lign. fig. 3_).

[442] See Foss. South D. p. 221, _et seq._ for further details.

Fig. 1.--Abdominal appendage, or tail, of Astacus[443] Leachii (G. A. M.),
with three abdominal segments.
2.--Part of the head of _Astacus Leachii_, with remains of the
squamous peduncles of the antennæ, and of the long setaceous
antennæ.
3.--Chelate claw, with part of the carpus, or wrist, of _Astacus
Leachii_.
4.--Chelate claw of Astacus[443] Sussexiensis (G. A. M.).

[443] These fossil Lobsters are now to be referred to Prof. M’Coy’s genus Enoploclytia.

Enoploclytia Sussexiensis. _Lign. 169, fig. 4._--The claws of this species are readily distinguished from the former by their broader and stronger proportions, and spinous character; the pincers are strong and tuberculated, and the moveable finger is more curved and shorter than its opponent. The entire crust of this lobster is muricated, or beset with spines and sharp tubercles.[444]

[444] Foss. South D. tab. xxx. _fig._ 3. In the beautiful work of the late Mr. Dixon, on the Fossils of Sussex, there are figures of very fine specimens of these two species of Enoploclytia (plate xxxviii*.) with detailed descriptions by Prof. Bell (p. 344); but unfortunately, from inadvertence, no reference is made to my former drawings and descriptions of these fossil Lobsters of the Chalk, and new names are given by the Professor; _E. Leachii_ appearing as "Palæastacus macrodactylus," and _E. Sussexiensis_ as "P. Dixoni." I need scarcely remind my readers that according to the established rules of Nomenclature the _earlier specific names_ must be preserved; and that, Prof. M‘Coy having already indicated the distinction of these fossils from the recent _Astacus_ by providing an appropriate generic appellation, these new names are altogether inadmissible.

Claws of other _Astacidæ_ have been found in the Chalk of Sussex and Kent; one species in particular is distinguished from those previously described by its short curved pincers, and granulated surface;[445] and remains of other species of _Enoploclytia_, from near Cambridge and Maidstone, are described by Prof. M’Coy.

[445] This species may be named _Astacus cretosus_, to indicate its geological habitat.

The "Lobster-clays" in the Lower Greensand, or Neocomian, beds at Atherfield afford numerous fine specimens of a small long-clawed crustacean, allied to _Astacus_; a fine specimen is figured in the _Geology of the Isle of Wight_, title-page vignette, and see page 232. One or more species, apparently distinct from those of Atherfield, have been discovered by Mr. Beckles in beds of clay that appear to occupy the line of junction between the Wealden and Greensand, on the Sussex coast, between Pevensey and Bexhill.

Imperfect claws of _Astacidæ_ have also been found at other places in the Shanklin Sand; and I have collected from the Galt at Ringmer, the abdominal segments of a small species, resembling _Meyeria ornata_, _Lign. 170_.

Meyeria ornata: _nat._ (_Phillips and M’Coy._)]

The carapaces of two or three small _Astacidæ_, sometimes with the abdominal segments attached, as in the beautiful fossil figured in _Lign. 170_, are found in the Speeton Clay, near Scarborough. In this example the post-abdomen and its appendages are entire, and traces remain of the antennæ and some of the feet. These specimens are commonly imbedded in masses of indurated clay, as the fern-leaves in the Carboniferous ironstone, and are discovered by splitting the nodules through their longest diameter. The carapace of another small species (_Glyphea rostrata_)[446] occurs in the Oolite of Scarborough. My friend, the Rev. J. B. Reade, has also discovered an example of this species in that rich repository of organic remains, the Kimmeridge Clay, at Hartwell, Bucks.

[446] This is the _Astacus rostratus_ of Prof. Phillips’s Geol. York. vol. i. tab. 4, _fig._ 20.

A remarkable macrurous crustacean (_Eryon Cuvieri_) is found in the Jurassic limestone of Solenhofen. Perfect specimens of this species are occasionally seen in collections; it is distinguished by its very large, flat, oval shell, with the front lateral margins strongly dentated, and by its short setaceous antennæ; the front claws are as long as the body, and armed with pincers; the post-abdomen consists of six segments, terminating in a caudal appendage or tail.

In the United States several fossil crustaceans have been noticed in the Cretaceous strata of New Jersey; some of which are said to be related to _Pagurus_, and others to _Astacus_.

Fossil Prawns and Shrimps, of exquisite beauty, are found in the lithographic limestone of Pappenheim: a specimen from that locality (_Palæmon spinipes_) is figured _Wond._ p. 513: see also Frontispiece of this work.

A large crustacean of the Shrimp family has been discovered by the Earl of Enniskillen in the Lias of Lyme Regis.[447] Other specimens also of Macrura, more or less perfect, have been obtained from the same rich mine of organic remains: especially some in which the _branchiæ_, or respiratory organs, remain; and a portion of the post-abdomen, or tail, of a Cray-fish, as large as the common species.

[447] This beautiful fossil is figured and described by Mr. Broderip, Geol. Trans, second series, vol. v. pl. xii. under the name of _Coleia antiqua_.

[Sidenote: FOSSIL ISOPODOUS CRUSTACEANS.]

Isopodous Crustaceans. _Lign. 171._--Isopoda (_equal-feet_) is the term applied to an order of crustaceans in which the body is composed of a distinct head, and seven rings, each having a pair of equal feet; the common _Oniscus_, or wood-louse, is a familiar example of a terrestrial _Isopod_. This order includes many genera and species, some of which nearly approach the extinct family of crustaceans (Trilobites) whose remains abound in the palæozoic strata; and the parasitical Isopod, _Bopyrus_, that infests the common Prawn, is closely related to certain genera of Trilobites, hereafter described.

Impressions of the upper and under surfaces.]

One species of fossil _Isopod_ has been found in green fissile marl, at Montmartre; and another in fine-grained limestone, probably, from Pappenheim.[448] No fossil remains of this order had been noticed in the British strata, until the discovery of the _Archæoniscus_ in the Purbeck strata by the Rev. P. B. Brodie. The quarry in which these relics were found is situated at Dallards, near the village of Dinton, about twelve miles west of Salisbury. They are principally distributed in a bed of light-brown and grey limestone, in the lower part of which are numerous fresh-water bivalves (_cyclades_), and a few small oysters. These Isopods often occur in clusters (see _Lign. 171_); the lenses of the compound eyes are sometimes detectable in the limestone, and, rarely, attached to the head; traces of legs have been observed, but no antennæ. In the same stratum the _elytron_ (sheath, or wing-case) of a coleopterous insect was discovered. Mr. Brodie has obtained specimens an inch and a half in length, and an inch broad. These fossils appear to have been deposited tranquilly at the bottom of the water which they inhabited, since they are usually found imbedded with their legs downwards, and generally well preserved.[449]

[448] Nat. Hist. Crust. Foss. p. 138.

[449] See Brodie’s Fossil Insects, pp. 3 and 10.

The _Archæonisci_ also occur in the Purbeck insectiferous limestone of Durlstone Bay, near Swanage, and have been discovered in strata of a similar character at the Ridgway railway-cutting between Dorchester and Weymouth, by the Rev. Osmond Fisher, of Dorchester.

Entomostraca.--The Crustaceans that we have above noticed belong to the Sub-class Malacostraca; and we have now to describe some fossil genera belonging to various divisions of the Entomostracous Crustaceans.

_Carb._ _Coalbrook Dale._]

Fig. 1. Limulus Rotundatus, the imprint of _fig._ 3.
2.--The nodule before it was broken.
3.--Limulus Rotundatus, (the corresponding portion of the nodule,
_fig._ 1,) showing the entire form of the carapace.

[Sidenote: FOSSIL LIMULUS.]

The Limulus (Mollucca or King Crab) is a genus belonging to that Division of the Entomostraca termed Pœcilopoda and is abundant in the seas of India and America. The carapace is crustaceous and of a semilunar form, the head and thorax are blended together, and the superior abdominal shield, which is composed of confluent segments, appears like one piece, and has an indistinct trilobed character; the last segment is prolonged into a three-edged, sharp, styliform weapon. The Limulus has two reniform, compound eyes, composed of facets of a peculiar form. The gills are disposed on lamelliform processes. It is distinguished from all other crustaceans by the mastication of its food being performed by the first joint of the thoracic legs which surround the mouth, instead of by jaws.

_Coal Measures._ _Coalbrook Dale._]

Very fine examples of a fossil species of this remarkable genus are occasionally obtained from the lithographic stone of Solenhofen.[450] In England three small species have been discovered in nodules of ironstone and indurated clay, in the Carboniferous strata of Coalbrook Dale, by Mr. Prestwich.[451] In one example (_L. Anthrax_[452]) two of the legs are seen extending from under the body; in another species the sharp, pointed process of the tail is well developed (_Buckl._ p. 396, and tab. xlvi″.; see also _Lign. 173_). The specimen figured _Lign. 172_ is one of several examples which I obtained by breaking up nodules from that celebrated locality. _Fig. 2_ represents the nodule unbroken, and without any indication of its contents; by a well-directed blow it was separated into two equal portions, _figs._ 1 and 3, in which the carapace of the crustacean, and its marginal appendages, are well displayed. The rounded form of the carapace, and the membrane which appears to connect the spines, separate this species from all others.

[450] Hist. Crust. Foss. pl. xi. _fig._ 6.

[451] See Mr. Prestwich’s Memoir on the Geology of Coalbrook Dale, Trans. Geol. Soc. second series, vol. v. part 3.

[452] Trans. Geol. Soc. second series, vol. v. pl. xli. figs. 1-4.

The Eurypterus and Pterygotus are palæozoic crustaceans, of large size. They are regarded by Prof. M‘Coy and Mr. Salter as belonging to the Pœcilopoda, and as differing from the Limulus chiefly in having the segments of the abdomen freely articulating with each other.[453] The _Eurypterus_ was first described by Mr. Dekay, in the United States, probably from the Carboniferous system. The head is round, the thoracic and cephalic portions of the carapace being blended together, and the abdominal region is formed of eleven segments, with a caudal appendage. It has two depressed, lunated eyes, remote from each other, and eight feet, the anterior pairs furnished with _branchiæ_, and the hindmost pair relatively larger than in any other crustacean. Two American species are described; the one is five, and the other about four inches long.[454] In the Carboniferous strata at Burdie-house, near Edinburgh, and of Kirton, near Glasgow, a large species of this curious genius has been found by Dr. Hibbert and Dr. Scouler; the length of some specimens being estimated at from twelve to eighteen inches.[455]

[453] See Salter on Pterygotus, Quart. Journ. Geol. Soc. vol viii p. 387.

[454] Dr. Harlan, in Trans. Geol. Soc. Pennsylvania, vol. i. p. 96.

[455] See the elegant Memoir on the Fresh-water Limestone of Burdie-house, near Edinburgh, by Samuel Hibbert, M.D. F.E.S.E.

Other species of _Eurypterus_ have been noticed in the Upper Silurian rocks of Kendal, Westmoreland,[456] and of Kington, Radnorshire; and in the Devonian of Russia.

[456] Pal. Foss. Cambridge, Appendix.

Pterygotus.--In the Old Red sandstone of Forfarshire, and other parts of Scotland, the remains of this remarkable crustacean have been long known to the quarry-men by the name of "_petrified Seraphims_;" from an imaginary resemblance of the expanded post-abdomen to the usual representations of those ideal beings! This genus is characterised by the angular carapace, which forms a lozenge-shaped shield; and the appendage of the post-abdomen, which, instead of being divided into segments, as in most animals of this class, is a continuous flap. The eye-pits on the carapace are like those of Eurypterus, but are very large. The claws resemble those of the common lobster. The external crustaceous covering is ornamented with circular and elliptical markings, producing an imbricated or scaly appearance, the imprints of which gave rise to the enigmatical "Seraphims" of the Forfarshire sandstone. Some specimens indicate a total length of four feet![457]

[457] The Old Red Sandstone, or New Walks in an Old Field, by Hugh Miller, p. 147. There are specimens in the British Museum from the quarries of Carmylie.

Besides this Devonian species (_P. anglicus_), there is also a Silurian species (_P. problematicus_), from Herefordshire.[458]

[458] See Quart. Journ. Geol. Soc. vol. viii. p. 386.

* * * * *

Several fossil genera of the Entomostracous Crustaceans belong to the _Phyllopoda_ (_leaf-feet_), which constitute a subdivision of the Branchiopoda (_gill-feet_). Of these perhaps the most remarkable is the Dithyrocaris, first discovered by Dr. Scouler in the Carboniferous shale near Paisley. This genus is allied to the recent _Apus_, and, like it, has a broad, flat, thin carapace, easily divisible down the middle of the back, and a lengthened tail or post-abdomen, with a trifid termination. Six species have been found in the Carboniferous deposits of the British Isles.

Ceratiocaris and Hymenocaris, which are related to the recent _Nebalia_, are also of the Phyllopod group, and maybe said to resemble a shrimp-like animal partly enclosed in a bivalve carapace, while its tail is exposed, and either protruded, or turned beneath the body. Of the first-mentioned genus there are three Upper Silurian species; and of _Hymenocaris_ Mr. Salter has described a single species (_H. vermicanda_) from the Lower Silurian (Cambrian) of North Wales.

The _Limnadiadæ_,--another Phyllopodous family, of which the recent _Limnadia_ is the type,--are well represented in the older rocks. These Crustaceans are also bivalved, but the body is wholly enclosed. They are of small size. Estheria, a genus closely related to Limnadia, occurs plentifully in the Wealden of Sussex and Germany, and in the Lias of Westbury. Mr. Bean also has noticed a species (_E. concentrica_) in the Oolite shell of Gristhorpe Bay; and the _E. minuta_[459] is characteristic of the Keuper division of the Trias in England and Europe.

[459] This was formerly described as a _Posidonomya_.

Leperditia and Beyrichia are other fossil genera belonging to the _Limnadiadæ_. These little crustaceans were gregarious in their habit, and, like Estheria, occur locally in great profusion. They are characteristic of the Silurian deposits; the former being an abundant Upper Silurian fossil in Europe and North America, and the latter,--a very minute form,--both in the Upper and Lower Silurian.

The next group of Entomostraca that we have to notice belongs to the Lophyropoda (_feet crested with bristles_), namely, the Ostracoda (_shelled_). The recent genera, _Cypris_, _Cythere_, and _Cypridina_,[460] are types of the three families of this group, and are the existing representatives of numerous closely related forms that occur in a fossil state.

[460] The student should consult Dr. Baird’s elegant and elaborate work on the Natural History of the British Entomostraca (published by the Ray Society), for information on the characters and habits of these interesting little crustaceans and their numerous allies

Fig. 1.--Cypris Valdensis: _magn._; showing the ventral aspect of
the united valves. _Cooksbridge._
1_a_.--Side view of the same, _magn._ The three small figures are
of the _nat. size_.
2.--Cypris Fittoni, _magn._ _Langton Green._
3.--Cypris tuberculata, _magn._ _Langton Green._
3_a_.--View of the same, showing the lower edges of the valves
united: the three small figures, _nat._
4.--Cypris granulosa, _magn._; view showing the hinge-line.
5.--A piece of Weald Clay, with Cyprides and Shells; _nat._

Cypris. _Lign. 174._--The animals of this genus belong to those Crustaceans in which the covering of the body is not divided into transverse segments, but consists of a large dorsal shield, having the form of a bivalve shell. They are often very minute, and numerous kinds swarm in our lakes and pools. The species of an allied genus, named _Cythere_, the shells of which cannot always with certainty be distinguished in a fossil state from those of _Cypris_, inhabit salt or brackish waters. As the living _Cyprides_ are interesting objects for microscopic examination, they are commonly shown in the exhibitions of the hydro-oxygen microscope, and their appearance is therefore well known. Two recent species are figured in _Ly._ p. 183. These animals have the body enclosed in a bivalve, horny case, the two pieces being united by a hinge-line. They have four feet, and two pairs of antennæ, with a pencil of cilia at the extremities; and one compound eye. They swim with rapidity, by means of their ciliated antennæ, and crawl about actively on the water-weeds and other subaqueous surfaces. Like the other crustaceans, they frequently moult or cast their cases, and the surface of the mud spread over the bottoms of still lakes is often covered with their exuviæ. The largest living Cypris (_C. clavata_) does not exceed one-eighth of an inch in length. The fossil cases or shells of Cyprides are found in considerable variety and in prodigious numbers in certain Tertiary and Secondary strata, which appear to have been deposited by fresh-water; as, for example, in the lacustrine marls of Auvergne (_Ly._ p. 183), and the fluviatile clays and limestones of the south-east of England. They have not been observed in any decidedly marine beds; but Mr. Lonsdale discovered in the Chalk, by means of the microscope, cases of crustaceans, that belong to the genus _Cythere_ (_Ly._ p. 26, figs. 21-24), the recent species of which inhabit the sea. One or more species of Cypris have been observed in the older British Tertiaries,--by Sir C. Lyell, in Hordwell Cliff (_Geol. Trans._ 2d ser. vol. iii. p. 288), and by Mr. Prestwich at Hempstead Cliff (_Brit. Assoc._ 1846); but in the Pleistocene deposits the Cypris and its congeners abound, and of these Mr. R. Jones has enumerated nine species (_Annals Nat. Hist._ 2d ser. vol. iii.). In many districts on the Continent, the Eocene marls and clays abound in these remains. Some of the fresh-water Tertiary strata of France contain myriads of a Cypris (named _C. faba_, from its bean-like form) which was formerly supposed, but erroneously, to be identical with a species found in the Wealden. The laminated marls of Auvergne contain, between every layer, countless myriads of the shells of Cyprides, through a depth of several hundred feet; although each lamina of marl scarcely exceeds the thickness of paper. Near Clermont, the green _cypridiferous_ marls approach to within a few yards of the granite which forms the borders of that ancient basin (_Ly._ p. 184). In the eastern portion of the basaltic districts of India, Mr. Malcolmson has discovered two species of Cypris, associated with fresh-water univalves and bivalves.[461] In England the principal deposits of the Cyprides, are the clays and limestones of the Wealden and the Isle of Purbeck,[462] to the composition of which the relics of these minute beings have largely contributed. Entire layers of their cases are found in the laminated clays and marls on the southern shores of the Isle of Wight, at Atherfield and Sandown Bay, where some of the Wealden beds emerge from beneath the lower division of the Shanklin sand. Upon splitting the clay in the direction of the laminæ, the exposed surfaces are found to be covered with these minute bodies; as in the specimen, _Lign. 174, fig. 5_.

[461] Geol. Trans. 2d ser. vol. v. pl. xlvii.

[462] On account of the distinctive characters of their carapace-valves, M. Bosquet has suggested the appellation _Cypridea_ as a generic name for the Cyprididæ of the Wealden and Purbeck beds.

The appearance of four characteristic Wealden Cyprides, of their natural size and magnified, is shown in _Lign. 174_. The one named _C. Valdensis_, or Wealden Cypris, by Dr. Fitton and Mr. Sowerby (fig. 1), is the most frequent, and occurs in numerous localities in Kent and Sussex. A Cypris having the case studded with relatively large tubercles (fig. 3), is found in many of the finer sandstones of Tilgate forest; another, with the shell tuberculated, but divided by a transverse ridge (fig. 2), indicating a rudimentary condition of the segments which characterise the class, is certainly distinct from the former, and can have no name more appropriate than that of _C. Fittoni_. The other beautiful species (_C. granulosa_, fig. 4) has the surface of the case covered with granules. One more species has been observed in the Weald clay, at Sandown Bay and Atherfield (by Mr. Lonsdale); it is distinguished by a short conical spine on each valve, and is therefore named _C. spinigera_. In the Weald clay at Resting-oak-hill, near Cooksbridge (_Geol. S. E._ p. 187), _C. Valdensis_ is so abundant, that every thin flake is covered with its white calcareous shells; and upon breaking the nodules and septaria of reddish-brown ironstone which occur in that locality, myriads of beautiful sharp casts of the cases are observable in almost every fragment. They are associated with fresh-water bivalves and univalves (_Cyclas_ and _Paludina_), and minute scales of fishes. The sandstone at Langton Green, near Tunbridge Wells, which contains casts and impressions of several species of fresh-water shells, abounds in _Cyprides_; and the layers of argillaceous ironstone, interstratified with the sandstone in one of the quarries, are particularly rich in these remains. The surface of a recently broken slab is often covered by minute, polished, oblong, convex bodies, which are the casts of cypridean carapaces.

The Sussex marble is largely composed of the remains of these minute crustaceans. Upon examining thin polished slices of this limestone under the microscope, the cavities and interstices of the shells are found to be filled with the shields of Cyprides, entire or in fragments; and some specimens of the Purbeck marble equally abound in these remains. The Purbeck marls, as well as the limestones, often abound with Cyprides. According to Prof. E. Forbes, these belong to several species, all more or less distinct from those of the Wealden (_Brit. Association_, 1850). The Cyprides of the Wealden of Germany have been figured and described by Dr. Dunker; and several of the British forms are found there.

As the recent species inhabit still lakes, or gently running streams, and not the turbulent waters of estuaries, we cannot doubt that the strata in which these animals so largely predominate were deposited in lakes or bays, communicating with the river which transported to their present situation the bones and other remains of the colossal reptiles of the Wealden. And the beds of fresh-water snails, with scarcely any intermixture of other organic remains but the Cyprides, which are spread over extensive areas in the Wealden and Purbeck districts, appear to afford corroborative proof of this inference.

Four species of minute bivalved Entomostraca from the Carboniferous deposits have been referred to _Cypris_: viz. _C. arcuata_, Bean, from the Coal-shale at Newcastle; _C. inflata_, Murchison, Coal-measures, near Shrewsbury; _C. Scoto-Burdigalensis_, Hibbert, Coal-measures at Burdie-house, and Coal-shale at Derry; and _C. subrecta_, Portlock, also from Derry, Tyrone.

Cythere.[463]--This animal differs but little from Cypris except in having an additional pair of feet. In the Subgenus Cythereis the valves are thick, oblong, and strongly hinged; thus differing from the thin and more or less oval valves of the true Cythere and of Cypris. Several species of _Cythere_ and its sub-genera occur in the Tertiary, Cretaceous, Oolitic, Liassic, Permian, and Carboniferous deposits.[464]

[463] For description and illustration of this genus, see Baird’s British Entomostraca, p. 163, &c.

[464] Consult M. Bosquet’s Memoir on the Tertiary Entomostraca of Belgium and France; Mr. T. R. Jones’s Monograph of the Cretaceous Entomostraca (Palæontographical Society), and of the Permian Entomostraca, in Prof. King’s Monograph of the Permian Fossils (Palæont. Soc.); and Prof. M’Coy’s Synopsis of the Characters of the Mountain Limestone Fossils of Ireland.

Three species of Entomostraca, very closely related to Cypridina,[465] from the Carboniferous rocks of Belgium, hare been figured and described by M. De Koninck; one species from the Carboniferous rocks of Ireland, by Prof. M’Coy;[466] and two species from the Cretaceous limestone of Maestricht,[467] by M. Bosquet. The genera _Cyprella_ and _Cypridella_ have been established by M. De Koninck for the reception of some allied forms found in the Carboniferous strata of Belgium; and Entomoconchus (M‘Coy) and _Daphnoidia_ (Hibbert) are allied British Carboniferous forms.

[465] See Baird’s British Entomostraca, p. 176, &c.

[466] Under the name of _Daphnia primæva_.

[467] Under the generic appellation of _Cyprella_.

Trilobites.--Among the numerous petrifactions which are found in the limestones in the neighbourhood of Dudley, in Staffordshire, there are certain fossil bodies which, from their extraordinary form and appearance, have for more than a hundred and fifty years been objects of great interest to the naturalist, and of wonder to the general observer, and have long been provincially termed _Dudley insects_, or _locusts_.[468] By the earlier naturalists these fossils were referred to fishes, to molluscs, and to insects, before their real character was discovered. The most common type consists of a convex, oblong body, divided transversely into three principal parts, and longitudinally into three lobes, by two deep, parallel furrows; this last character suggested the name _Trilobita_, or _Trilobites_, by which the family is now distinguished by naturalists.

[468] Lhywd. Philos. Trans. for the year 1698.

Fig. 1.--Illænus perovalis. (_Murch. Sil. Syst._)
2.--Trinucleus Lloydii. (_Murch. Sil. Syst._)
3.--Calymene Blumenbachii: the upper surface of the
carapace, viewed from above. Dudley.
4.--The same, coiled up, and seen in profile.
4_a_.--The same, seen in front.
]

These fossils are the carapaces, or shells, of crustaceans, belonging to an extinct family,[469] which comprises many genera, and numerous species. Mortimer, Da Costa, Guettard, and Linnæus recognised the crustacean character of this interesting, but obscure class of organic remains: their true affinities, however, were first scientifically determined by Alex. Brongniart.[470] Many memoirs on the _Trilobites_ have since been published by eminent Continental and American writers;[471] and much light has been thrown on the subject by the labours of Martin,[472] Parkinson,[473] Stokes, Phillips,[474] König,[475] Dr. Buckland,[476] Sir E. I. Murchison,[477] and others. The beautiful illustrations and interesting description of the Trilobites in Dr. Buckland’s Bridgewater Treatise (_Bd._ p. 389, and pl. xlv. xlvi.) must have rendered the reader familiar with the most important facts relating to these extinct beings; but subsequent discoveries have thrown additional light on their structure and natural affinities. The works of M. Burmeister,[478] Mr. Salter,[479] Prof M’Coy[480] and especially of M. Barrande,[481] should be carefully referred to by the student in this branch of Palæontology.

[469] The Trilobites appear to have been related more nearly to the Phyllopoda than to any other division of the Crustacea.

[470] Hist. Nat. Crust. Foss. Burmeister on Trilobites; Introduction.

[471] A Monograph on the Trilobites of North America, by Jacob Green, M.D. Philadelphia, 1832.

[472] Petrificata Derbiensia.

[473] Organic Remains of a Former World, vol. iii. See Pictorial Atlas.

[474] Geology of Yorkshire; and Palæozoic Fossils of Devon.

[475] Icones Fossilium Sectiles.

[476] Bridgewater Treatise.

[477] Silurian System, chap, xlvii.

[478] Die Organisation der Trilobiten. 1843. Translation: Ray Society.

[479] In the Quarterly Journal of the Geological Society, and in the Memoirs and Decades of the Geological Survey of Great Britain.

[480] Palæozoic Fossils in the Cambridge Museum.

[481] The Silurian System of Bohemia: the Trilobites.

In the Trilobites the head is distinct, and without antennæ, and the feet are supposed to have been rudimentary, soft, and membranaceous: the essential characters which separate them from all other crustaceans, except _Bopyrus_ (a parasite on the branchiæ of the common prawn), are, according to Mr. Macleay, the deficiency of antennæ, and of lateral posterior abdominal appendages, and the presence of evanescent feet. Like other crustaceans, the Trilobites were subject to the process of metamorphosis during their early stages of life; and M. Barrande has ascertained that one species, the _Saö hirsuta_, appeal’s in no less than twenty different stages of development. In its earliest, embryonic condition, it is a simple disk, and it passes through various stages until it becomes a perfect adult trilobite, having seventeen free thoracic segments and two caudal joints. No less than ten genera and eighteen species were instituted by palæontologists on _some_ of the forms only which this one species presents in its different stages of metamorphosis, before M. Barrande’s laborious and long-continued investigations gave him an insight into the true relations of these various conditions of the same animal to one another. This talented and indefatigable palæontologist has arrived at like results with other Trilobites, and has been enabled to add greatly to our knowledge of the natural history and geological distribution of this interesting group of crustaceans. See Transact, of the Sections, Brit. Assoc. 1849 and 1850; and _Trilob. Bohême_, pl. vii.

The _Trilobites_ have been arranged in numerous genera, the names of which in a few cases are expressive of natural characters, but in others have reference to the obscurity that still invests some parts of the organization of these animals.[482]

[482] As, for example, Asaphus, _obscure_; Calymene, _concealed_; Agnostus, _unknown_.

Calymene Blumenbachii. _Lign. 175, figs. 3, 4._--This is the Trilobite so well known as the Dudley locust, or insect. It consists of an ovate, convex, trilobed crustaceous shell, or case, and is found either expanded, as in _Lign. 175, fig. 3_, with its under surface attached to, and blended with, the limestone (_Wond._ p. 789); or coiled up like an Oniscus, or wood-louse, as in _figs._ 4 and 4_a_. The head is large, convex, rounded in front, with a broad border, and divided into three lobes by two longitudinal depressions. The eyes are two in number, compound, and have numerous facets; they are situated on the sides of the head, remote from each other. The carapace is deeply trilobed by two longitudinal furrows; the thoracic portion is composed of thirteen segments; the caudal shield is small and nearly semicircular. This species is from one to four inches in length. It occurs from the Lower Llandeilo rocks up to the Upper Ludlow inclusive.

The structure here described may be regarded as the normal type, but numerous and important modifications prevail in the different genera.

(_Reduced from_ pl. vii. _Sil. Syst._) _Upper Silurian._ _Dudley._]

In the genus Homalonotus, _Lign. 176_, the thoracic portion of the carapace is but obscurely lobed, and consists of thirteen segments; the abdominal is distinct from the thoracic, and formed of nine rings; it terminates in a prolonged point. The _H. Herschelii_ is a large Trilobite, very plentiful in the Upper Silurian schists of the Cape of Good Hope.