Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/429

 AMPHIBIA

383

teristic of frogs in which, as development shows, the vertebra is formed wholly or for the greater part by the neural arch (13). Small forms from the Upper Carboniferous and Permian formations. A single family : Branchiosaurida;. II. Apoda (14).—No limbs. Tail vestigial or absent. Frontal bones distinct from parietals j 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 Csecilians merely a family of the Urodeles. This view has originated out of the very remarkable superficial resemblance between the Ichth?/ophis-la,Ya and the Amphiuma. Cope (15) 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 the cousins Sarasin (16), 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, Pig 2 —A Dorsal vertebra, B, Caudal vertebra of Archegosaurus. (Outline no objection to unite the Csecilians with the Urodeles; after Jaekel.) na, neural arch; ch, chorda; pi, pleurocentrum; ic, mtercentruin. but, to say nothing of the scales, present in many genera chevrons in the caudal region. Mostly large forms, of Carboni- of Apodals and absent in all Caudatee, which hawe ferous and Permian age, with a more or less complex infolding of been shown by Credner to be identical in structure with the walls of the teeth. Families : Ak.chegosaurid2ES, Eryofid*;, those of Stegocephalians, the Csecilian skull presents Trimerorhachid^j, Dissorhophid^e. The last is remarkable for features which are not shared by any of the tailed an extraordinary endo- and exo - skeletal carapace, Dissorhophus batrachians. G. M. Winslow (17), who has made a being described by Cope (12) as a “ batrachian armadillo. ” B. Embolomeri, with the centra and intercentra equally study of the chondrocranium of Ichthyodeveloped discs, of which there are thus two to each neural arch ; phis, concludes that its condition could these discs perforated in the middle for the passage of the notochord. This type may be directly derived from the preceding, not have been derived from a Urodele with which it appears to be connected by the genus Diplospondylus. form, but points to some more primitive ancestor. That this ancestor was Fam. : Cricotid^e, Permian. C. Labyrinthodonta, with simple biconcave vertebral discs, very nearly related to, if not one of the slightly pierced by a remnant of the notochord and supporting the Stegocephalians, future discovery will loosely articulated neural arch. This condition is derived from in all probability show. that of the Rhachitomi, as shown by the structure of the vertebral III. Caudata (18).—Tailed batrachcolumn in young specimens. Mostly large forms from the Inas (a few Permian), with true labyrinthic dentition. Families : ians, with the frontals distinct from LaBYRINTHODONTIDJS, ANTHRACOSATTRIDiB, DENDRERPEUDiE, the parietals and the palatines from Nyraniid^:. ., , . , , D. Microsauria, nearest the reptiles, with persistent notochord the maxillary. Some of the forms 1 J surrounded by constricted cylinders on which the neural breathe by gills throughout their existcompletely arch rests. Teeth ence, and were formerly regarded as hollow, with simple establishing a passage from the fishes or only slightly folded walls. Mostly to the air-breathing batrachians. They of small size and are now considered as arrested larvae abundant in the Car- descended from the latter. One of the boniferous and Lower most startling discoveries of the decade Permian. Families: U ROCORDYLID B&, 1890-1900 was the fact that a number Limnerpetid^:, of forms are devoid of both gills and Hylonomid^e, (Fig. lungs, and breathe merely by the skin 3) Microbrachid.e, and the buccal mucose membrane (19). Dolichosomatid,® ; Fig. 4.—Typhlomolge rathbuni. the latter serpenti- Three blind cave-forms are known : one terrestrial—Typhlotriton, from North form, apodal. E. Branchiosauria, America, and two perennibranchiate—Proteus in Europe nearest to the true and Typhlomolge (Fig. 4) in North America. batrachians ; with persistent, non-conThis order contains about 150 species, referred to five families : stricted notochord, Hyl#;obatrachida:, Salamandrida:, Amphiumida:, Proteida:, surrounded by Sirenida). „, t barrel-shaped, bony Fossil remains are few in the Upper Eocene and Miocene ot cylinders formed by Europe and the Upper Cretaceous of North America. The oldest the neural arch Urodele known is Eylccobatrachus, Dollo (20) from the Lower Fig. 3.—A, Dorsal vertebra of Hylonomus (side view above and a pair of Wealden of Belgium. At present this order is confined to tne and front view). B, Dorsal vertebra of Branchio- intercentra below, northern hemisphere, with the exception of two SpelerpesJrom 110 saurus (side view and front view). (After Credner.) both these elements Andes of Ecuador and Peru, and a Plethodon from Argentina. n, neural canal; ch, chorda. taking an equal IV. Ecaudata (21).—Frogs and toads. Four limbs and share in the formation of a transverse process on each side for the aupporlTof the rib. Thie plan of structure apparenly-ol™! out no tail. Radius confluent with ulna, and tibia with fibula ; of the rhachitomous type by suppression .^ tarsus (astragalus and calcaneum) elongate, forming an the downward extension of the neural arch, leads to that chaiac

A. Rhachitomi (Figs. 1, 2), in which the spinal cord rests on the notochord, which persists uninterrijpted and is surrounded by three bony elements in addition to the neural arch : a so-called pleuroeentrum 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