Diversity in the Scutes of Chelonia, 5 



nificance attached to normal scutes. Some representative views arc- 

 given in the next section. 



Some Recent Views regarding the Phylo genetic Significance of 



Normal and Abnormal Scutes. 



O. P. Hay's view as to the origin of the carapace (Hay, '98) is 

 quite important in this connection, for he seems to have been the 

 first to realize the probable phylogenetic significance of the epi- 

 deiinal scutes. Previous discussions had centered chiefly about the 

 dermal and periosteal bony skeleton. 



Hay distinguishes three kinds of bone in the carapace: (1) car- 

 tilage bone; (2) true dermal bone, developed in the skin itself, and 

 comparable to the osteodermal plates of Dermochelys and of the 

 crocodiles; (3) fascia bone, originating subcutaneously by ossifica- 

 tion of the fascia below the skin. The present carapace owes its 

 phylogenetic origin to the complete fusion of cartilage and fascia 

 bone. Tnie dermal bone has probably completely or almost com- 

 pletely disappeared from the carapace of modern Thecophora. But, 

 according to Hay, the ancestors of all turtles, Thecophora, as well as 

 Athechffi, have possessed an armor of true dermal bony plates, 

 probably mosaically arranged, and with twelve well differentiated 

 keels. Of the keels, five were dorsal (one median and two pairs 

 lateral), two were marginal, and five ventral (one median and two 

 pairs lateral). The plates of this armor were adapted to previously 

 formed epidermal scutes of the same mosaic pattern. This osteo- 

 dermal armor is retained in Dermochelys with essentially the primi- 

 tive pattern, but the epidermal shields are lost in the adult and 

 indicated only in the young. 



In Thecophorous turtles the mosaic 12-keeled dermal armor under- 

 went important modifications partly in correlation with the greater 

 development of the internal skeleton. The plates became very much 

 reduced in number, a few of the plates of the keels, growing in size, 

 and assuming the whole function of the protective ai-mor, crowded 

 out many of the keel plates, and all of the smaller plates between the 

 keels. Some of the keels too were lost. As the result, we may sup- 

 pose a dermal carapace of twelve series of scute-covered plates, with 



