198 ANATOMY OF VERTEBRATES. 



scale, in the way that tiles are pegged together in the roof of a 

 house. 



In the Porcupine-fishes (Diodon) the spines are supported by 

 triradiate interlocking dermal bones. 



44. Dermoskeleton of Reptiles. --In the Scincoid family of the 

 Lacertians, the scales are more or less ossified ; least so in the 

 smooth-scaled genera (Scincus, Tiliqua) ; but in Cyclodus resembling 

 scutes, and giving a knobby character to the surface. In Cyclura, 

 Lophura, and Xiphosurus velifer, dermal bones in the form of 

 spines project or raise the skin above the dorsal or caudal vertebrae. 

 The horizontal plates connate with the neural spines, and with 

 the ribs, are dermal ossifications, as are the neural plates and 

 marginal plates which remain distinct from the endoskeleton, 

 in the composition of the carapace of the Chelonia. The plastron 

 is also formed by dermal plates, connate with the sternum and 

 sternal ribs. 



In existing Crocodilia the upper surface of the trunk is de- 

 fended by bony scutes, usually quadrate in form, smooth on the 

 inner surface, sculptured and longitudinally ridged on the outer ; 

 arranged in transverse series, more or less apart, of twos or 

 fours, upon the neck ; but six or eight in a transverse line and 

 close set, so as to have a longitudinal as well as transverse 

 arrangement along the back. The numbers and patterns of these 

 scutes are noted in zoological comparisons and characters of 

 genera and species. 1 The Alligators are defended by a ventral 

 as well as dorsal cuirass, separated, as Natterer observed in 

 Champsa palpebrosa, Ch. trigonata, Ch. gibbiceps, only by a 

 narrow and soft longitudinal groove along the sides of the neck 

 and trunk. 2 But the most remarkable anatomical modifications 

 are presented by the extinct and especially the mesozoic Cro- 

 codilia. 



The presence of a ventral as well as a dorsal series of scutes 

 and their distinctive characters were first noted in the Teleosauri. 

 The dorsal scutes are in close-set sub-imbricate transverse rows, 

 the posterior margin overlapping the anterior one of the next 

 row. I counted twenty such rows in a specimen of the Whitby 

 Teleosaur, of which sixteen covered the vertebrae between the 

 last cervical and first caudal. In the ventral shield or plastron, 

 only the two scutes in each row are on the same transverse parallel 

 which border the mid-line of the abdomen ; the others have an 

 alternate interlocking arrangement. These ventral scutes are 



1 CLI. torn. v. pp. 79, 80, pi. ir. 2 CLVH. torn. n. (1840) p. 320. 



