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.
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.
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| Fig. 2.—A, Dorsal vertebrae. B, Caudal vertebra of Archegosaurus. na, Neural arch; ch, chorda; pl, pleurocentrum; ic, intercentrum.
(Outline after Jaekel.) |
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.
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| Fig. 3.—A, Dorsal vertebra of Hylonomus (side view and front view). B, Dorsal vertebra of Branchiosaurus (side view and front view). n, Neural canal; ch, chorda. (After Credner.) |
II. Apoda (15).—No limbs. Tail vestigial or absent. Frontal bones distinct from parietals; palatines fused with maxillaries. Male with an intromittent copulatory organ. Degraded, worm-like batrachians of still obscure affinities, inhabiting tropical Africa, south-eastern Asia and tropical America. Thirty-three species are known. No fossils have yet been discovered. It has been attempted of late to do away with this order altogether and to make the Caecilians merely a family of the Urodeles. This view has originated out of the very remarkable superficial resemblance between the Ichthyophis-larva and the Amphiuma. Cope (16) regarded the Apoda as the extremes of a line of degeneration from the Salamanders, with Amphiuma as one of the annectent forms. In the opinion of P. and F. Sarasin (17), whose great work on the development of Ichthyophis is one of the most important recent contributions to our knowledge of the batrachians, Amphiuma is a sort of neotenic Caecilian, a larval form become sexually mature while retaining the branchial respiration. If the absence of limbs and the reduction of the tail were the only characteristic of the group, there would be, of course, no objection to unite the Caecilians with the Urodeles; but, to say nothing of the scales, present in many genera of Apodals and absent in all Caudates, which have been shown by H. Credner to be identical in structure with those of Stegocephalians, the Caecilian skull presents features which are not shared by any of the tailed batrachians. G. M. Winslow (18), who has made a study of the chondrocranium of Ichthyophis, concludes that its condition could not have been derived from a Urodele form, but points to some more primitive ancestor. That this ancestor was nearly related to, if not one of, the Stegocephalians, future discovery will in all probability show.
III. Caudata (19).—Tailed batrachians, with the frontals distinct from the parietals and the palatines from the maxillary. Some of the forms breathe by gills throughout their existence, and were formerly regarded as establishing a passage from the fishes to the air-breathing batrachians. They are now considered as arrested larvae descended from the latter. One of the most startling discoveries of the decade 1890–1900 was the fact that a number of forms are devoid of both gills and lungs, and breathe merely by the skin and the buccal mucose membrane (20). Three blind cave-forms are known: one terrestrial—Typhlotriton, from North America, and two perennibranchiate—Proteus in Europe and Typhlomolge in North America.
This order contains about 150 species, referred to five families: Hylaeobatrachidae, Salamandridae, Amphiumidae, Proteidae, Sirenidae.
Fossil remains are few in the Upper Eocene and Miocene of Europe and the Upper Cretaceous of North America. The oldest Urodele known is Hylaeobatrachus Dollo (21) from the Lower Wealden of Belgium. At present this order is confined to the northern hemisphere, with the exception of two Spelerpes from the Andes of Ecuador and Peru, and a Plethodon from Argentina.
IV. Ecaudata (22).—Frogs and toads. Four limbs and no tail. Radius confluent with ulna, and tibia with fibula; tarsus (astragalus and calcaneum) elongate, forming an additional segment in the hind limb. Caudal vertebrae fused into a urostyle or coccyx. Frontal bones confluent with parietals.
This order embraces about 1300 species, of which some 40 are fossil, divided into two sub-orders and sixteen families:—
A. Aglossa,—Eustachian tubes united into a single ostium pharyngeum; no tongue. Dactylethridae, Pipidae.
B. Phaneroglossa,—Eustachian tubes separated; tongue present. Discoglossidae, Pelobatidae, Hemiphractidae, Amphignathodontidae, Hylidae, Bufonidae, Dendrophryniscidae, Cystignathidae, Dyscophidae, Genyophrynidae, Engystomatidae, Ceratobatrachidae, Ranidae, Dendrobatidae.
The Phaneroglossa are divided into two groups; Arcifera and Firmisternia, representing two stages of evolution. The family characters are mainly derived from the dilatation or non-dilatation of the sacral diapophyses, and the presence of teeth in one or both jaws, or their absence. The Discoglossidae are noteworthy for the presence of short ribs to some of the vertebrae, and in some other points also they approach the tailed batrachians; they may be safely regarded as, on the whole, the most generalized of known Ecaudata. Distinct ribs are present at an early age in the Aglossa, as discovered by W. G. Ridewood (23). The recent addition of a third genus of Aglossa, Hymenochirus (24) from tropical Africa, combining characters of Pipa and Xenopus, has removed every doubt as to the real affinity which connects these genera. Hymenochirus is further remarkable for the presence of only six distinct pieces in the vertebral