76 VERTEBRATA. 



twisted except in a series of side coils. The body, clothed with scales, 

 is extremely elongated, chiefly by the multiplication of dorsal vertebrae. 

 There are no distinctive cervical and lumbar vertebrae. 



AH Ophidian fossils belong to the typical group (Colubridse). 

 Their remains are vertebrae, eggs and fangs. The Eocene clay of 

 Sheppey has yielded the earliest evidence of a Serpent, — the vertebrae 

 of the Palaeophis toliapicus. Species of the constricting, colubrine, 

 and venomous families existed before any of the living species of Mam- 

 malia; and there was the same adaptation to a prone posture and a 

 gliding movement with the belly in the dust as at the present day. 



Order 4 — Amphibia. 



Amphibians, the lowest members of the Class, are distinguished by 

 a skull depressed at the expense of the cranium, two occipital condyles 

 (the G-anocephala have no condyles), ribs absent or rudimentary, and 

 the body naked or very rarely (as in the ichthyoid species) covered with 

 very small embedded scales. They have no fixed type of external form. 

 The size, shape and number of teeth vary much ; sometimes the teeth 

 are wanting. The location of teeth on the vomerine bones is the only 

 dental character in which Amphibians differ from other Reptiles. It is 

 only in this Order among existing Reptiles that examples are found of 

 two or more rows of teeth on the same bone, especially on the lower jaw. 

 The vertebrae are concavo-convex (as in the Frog), or convexo-concave 

 (as in the Pipa), or biconcave (as in the Siren). The snake-like Coe- 

 cilians have no legs ; the tailless Ranadse have four ; and the tailed 

 Sal amandr idee two or four. 



Of the Amphibious Reptiles, the scaly Ganocepliala appeared first, 

 the Archegosaurus having been found in the Coal Measures. The 

 Labyrinthodonts were introduced in the same period, but did not attain 

 their full development till the Trias. These two extinct groups char- 

 acterize the transitional period between the Palaeo and Mezo-zoic 

 epochs. They lived along with the Ganoid Fishes. The soft-skinned 

 Batrachians belong to the age when most Fishes have the flexible cy- 

 cloid or ctenoid scales — namely, the Tertiary and Post-Tertiary. Frogs 

 and Salamanders have been discovered as low as the Miocene; and 

 Toads in the Pliocene. Tailed Batrachians are now on the decline ; the 

 tailless forms are most numerous and various to-day. 



