EXOSKELETON 



33 



These denticles are developed from connective-tissue cells, and are 

 not comparable to theplacoid denticle; the resemblance, too, between 

 the scales of the Dipnoi and Teleostei is only a superficial one. 



Thus the exoskeleton plays an important part in Fishes, and 

 in numerous fossil Amphibians it reached a still higher develop- 

 ment (Stegocephala). Amongst these, specially strong dermal 

 plates were formed in the region of the shoulder-girdle, and very 

 commonly most of the body was covered with scales. Fossil genera 

 of Amphibia have, however, bequeathed but slight traces of this 

 strong dermal armour to the existing forms of the group: as 

 examples may be mentioned the bony plates in the skin of the 

 back of certain Anura (Ceratophrys dorsata and Ephippifer auran- 

 tiacus), as well as the scales lying between the ring-like scutes of 



Fig. 21. — A, Carapace, and B, Plastron or \ Young Testudo Gr^;ca ; C, 

 Plastron of Chelone Midas. 



XX, neural plates ; C, O, costal plates ; MM, marginal plates ; Xp, nuchal 

 plate ; Py, Py, pygal plates ; E, entoplastron ; Ep, epiplastron ; Hy, hyoplas- 

 tron ; Hp, hypoplastron ; Xi, xiphiplastron ; P, S, ribs. ( V indicates the 

 anterior, and H the posterior end. ) 



the footless Amphibia (Gymnophioua). The latter resemble in 

 many points the scales of Fishes and Dipnoans, and maybe derived 

 from such a scaly covering as that of the Permian Salamander, Dis- 

 cosaurus. 



The dermal skeleton was still more highly developed amongst 

 fossil Reptiles, e.g., many Ornithoscelida (Stegosaurus). In these, 

 enormous bony plates and spines, sometimes as much as sixty-three 

 centimetres long, were present in the dorsal region. Teleosaurus 

 also, as well as the Triassic Aetosaurus ferratus and the Cretaceous 

 Nodosaurus textilis, possessed a strong exoskeleton. Amongst 

 existing Reptiles (comp. p. 20), Crocodiles, many Lizards 

 (Anguis, Cyclodus, Scincus), and more especially Chelonians, 

 exhibit a well-developed dermal skeleton. In the latter Order 



T) 



