42 COMPARATIVE ANATOMY 



from cells and corresponding to the dentinal layer of the basal 

 plate, can be distinguished from a deeper part composed of several 

 layers of connective tissue : each of these becomes independently 

 ossified in a typical manner. 



Numerous other forms of the dermal skeleton are met with in 

 Teleosts. In some of these Fishes (e.g. Plectognathi, Lopho- 

 branchii, certain Siluroids), 1 as in many of the earliest Paleozoic 

 Vertebrates (Ostracodermi), bony scutes are developed and form a 

 strong cuirass. In others, again (e.g. many Siluroids and Eels), the 

 scales may be reduced or absent. The bony dermal fin-rays or 

 " leptotrichia " of Teleostomes possibly correspond to modified 

 scales. 



Dipnoans. In the Dipnoi, as in the Teleostomi, the scales 

 are not directly derivable from the Elasmobranch placoids. In 

 form, in their overlapping arrangement, and in their situation in 

 pockets of the derm, they resemble the cycloid scales of Teleosts ; 

 but this similarity must have come about independently in the 

 two cases, that is, must be due to convergence. 



Amphibians. Recent Amphibians have retained only very 

 slight traces of such a dense integumentary bony armour as was 

 present in the fossil Stegocephali. 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. A series 

 of oblique and bilaterally symmetrical rows of scales covered the 

 entire ventral surface between the shoulder- and hip-girdles, a 

 further differentiation of which results in the scales no longer 

 overlapping, but forming short parallel rods, which correspond 

 to the so-called " abdominal ribs " (" parasternal elements ") of 

 certain Reptiles. As examples of exoskeletal structures in exist- 

 ing forms may be mentioned the bony plates in the skin of the 

 back of certain Anura (Ceratophrys dorsata and Brachycephalus 

 ephippium), as well as the scales lying between the ring-like folds 

 of the limbless Amphibia (Gymnophiona). The latter resemble 

 in many respects the scales of Fishes, and may be derived from 

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

 cosaurus. 



Reptiles. The dermal skeleton was very highly developed 

 amongst fossil Reptiles, e.g. certain Dinosaurs, such as the Jurassic 

 Stegosauridae, in which enormous bony plates and spines covered 

 with horn, sometimes as much as sixty-three centimetres long, 

 were present in the dorsal region. Teleosaurus and Aetosaurus 

 (Crocodilia), as well as some of the gigantic Cretaceous Dinosaurs, 

 (Ceratopsidoe), possessed a strong exoskeleton. 



Amongst existing Reptiles a series of well-developed "abdo- 



' In these Siluroids, the bony plates may bear sockets in which denticles, 

 consisting of dentine and enamel, are implanted. But although denticles are 

 retained in these cases, they do not contribute to the formation of the basal plates, 

 as in Elasmobranchs. 



