Chapter I: STEGOCEPHALIA (10).--Tailed, lacertiform or serpentiform

batrachians, with the temporal region of the skull roofed over by
postorbital, squamosal, and supratemporal plates similar to the same
bones in Crossopterygian fishes, and likewise with paired dermal bones
(occipitals and post-temporals) behind the parietals and
supratemporals. A parietal foramen; scales or bony scutes frequently
present, especially on the ventral region, which is further protected
by three large bony plates--interclavicle and clavicles, the latter in
addition to cleithra.

Extinct, ranging from the Upper Devonian to the Trias. Our knowledge
of Devonian forms is still extremely meagre, the only certain proof of
the existence of pentadactyle vertebrates at that period resting on
the footprints discovered in Pennsylvania and described by O.C. Marsh
(11) as _Tinopus antiquus_. Sundry remains from Belgium, as to the
identification of which doubts are still entertained, have been
regarded by M. Lohest (12) as evidence of these batrachians in the
Devonian. Over 200 species are now distinguished, from the
Carboniferous of Europe and North America, the Permian of Spitsbergen,
Europe, North America and South Africa, and the Trias of Europe,
America, South Africa, India and Australia. The forms of batrachians
with which we are acquainted show the vertebral column to have been
evolved in the course of time from a notochordal condition with
segmented centra similar to that of early bony ganoid fishes (e.g.
_Caturus_, _Eurycormus_), to biconcave centra, and finally to the
socket-and-ball condition that prevails at the present day. However,
owing to the evolution of the vertebral column in various directions,
and to the inconstant state of things in certain annectent groups, it
is not possible, it seems, to apply the vertebral characters to
taxonomy with that rigidity which E.D. Cope and some other recent
authors have attempted to enforce. This is particularly evident in
the case of the Stegocephalians; and recent batrachians, tailed and
tailless, show the mode of articulation of the vertebrae, whether
amphicoelous, opisthocoelous or procoelous, to be of but secondary
systematic importance in dealing with these lowly vertebrates. The
following division of the Stegocephalians into five sub-orders is
therefore open to serious criticism; but it seems on the whole the
most natural to adopt in the light of our present knowledge.

(Outline after Jaekel.)]

A. Rhachitomi, (figs. 1, 2), in which the spinal cord rests on the
notochord, which persists uninterrupted and is surrounded by three
bony elements in addition to the neural arch: a so-called
pleurocentrum on each side, which appears to represent the centrum
proper of reptiles and mammals, and an intercentrum or hypocentrum
below, which may extend to the neural arch, and probably answers to
the hypapophysis, as it is produced into chevrons in the caudal
region. Mostly large forms, of Carboniferous and Permian age, with a
more or less complex infolding of the walls of the teeth. Families:
ARCHEGOSAURIDAE, ERYOPIDAE, TRIMERORHACHIDAE, DISSORHOPHIDAE. The last
is remarkable for an extraordinary endo- and exo-skeletal carapace,
_Dissorhophus_ being described by Cope (13) as a "batrachian
armadillo."

B. Embolomeri, with the centra and intercentra equally developed
disks, of which there are thus two to each neural arch; these disks
perforated in the middle for the passage of the notochord. This type
may be directly derived from the preceding, with which it appears to
be connected by the genus _Diplospondylus_. Fam.: CRICOTIDAE, Permian.

C. Labyrinthodonta, with simple biconcave vertebral disks, very
slightly pierced by a remnant of the notochord and supporting the
loosely articulated neural arch. This condition is derived from that
of the _Rhachitomi_, as shown by the structure of the vertebral column
in young specimens. Mostly large forms from the Trias (a few Permian),
with true labyrinthic dentition. Families: LABYRINTHODONTIDAE,
ANTHRACOSAURIDAE, DENDRERPETIDAE, NYRANIIDAE.

D. Microsauria, nearest the reptiles, with persistent notochord
completely surrounded by constricted cylinders on which the neural
arch rests. Teeth hollow, with simple or only slightly folded walls.
Mostly of small size and abundant in the Carboniferous and Lower
Permian. Families: UROCORDYLIDAE, LIMNERPETIDAE, HYLONOMIDAE (fig. 3),
MICROBRACHIDAE, DOLICHOSOMATIDAE, the latter serpentiform, apodal.

E. Branchiosauria, nearest to the true batrachians; with persistent
non-constricted notochord, surrounded by barrel-shaped, bony cylinders
formed by the neural arch above and a pair of intercentra below, both
these elements taking an equal share in the formation of a transverse
process on each side for the support of the rib. This plan of
structure, apparently evolved out of the rhachitomous type by
suppression of the pleurocentra and the downward extension of the
neural arch, leads to that characteristic of frogs in which, as
development shows, the vertebra is formed wholly or for the greater
part by the neural arch (14). Small forms from the Upper Carboniferous
and Permian formations. A single family: BRANCHIOSAURIDAE.

(After Credner.)]