"ABNORMALITIES" IN CHELONIA. 75 



of the tubercles seen on the dorsal ridge of the tail of Clielydra 

 scrpcntina, and this suggests that the ossicles of Toxochelys are 

 merely a continuation forward of a series of tubercles that must 

 have been present on the tail. 



Hay suggests that the keels seen especially in the young of 

 modern Chdonia are the representatives of ancient dermal tuber- 

 cles that formed the chief armor of ancestral forms. That in 

 most cases these dermal ossicles have ceased to form indepen- 

 dently of the deeper and more vigorous bony layers is perhaps 

 to be expected as the result of condensation in developmental 

 processes. 



The degree to which modern species exhibit keels is extremely 

 varied. Some highly specialized forms show none, or at most 

 one, even in very young specimens, while one very primitive spe- 

 cies, Macrochclys tcmmincki, possesses seven distinct keels on the 

 carapace and four rows of flat scutes on the plastron. This mul- 

 tiplicity of keels is evidently a very primitive condition and natu- 

 rally suggests to Hay the condition seen in Dermochelys coriacea 

 in which twele well-marked keels are found, each keel consist- 

 ing of rows of dermal ossifications that are lager and more 

 prominent than the remaining intermediate ossicles that form the 

 continuous pavement of the test. This peculiar aberrant che- 

 lonian is taken by Hay, following Baur and others, as the hypo- 

 thetical ancestral type from which our modern chelonians have 

 been derived by a process of simplification. 



A survey of the field reveals the fact that the nearest approach 

 to this condition of twelve rows of keels is seen in Macrochelys 

 tcmmincki, which possesses seven distinct keels on the carapace. 

 The four rows of flat scutes on the plastron may once have been 

 keeled, for keels on the plastron are known in both extinct and 

 living groups. The total number of keels or keel equivalents in 

 Macrochelys is then eleven, one short of the supposed ancestral 

 condition. The missing keel is the mid-ventral one and is repre- 

 sented in certain groups by intergulars. Thus all of the ancestral 

 keels find representatives among modern species. 



Hay seems to have been the first observer to suggest the im- 

 portance of the scutes as factors in the evolution of the carapace. 

 Previous authors have confined their attention to the bony struc- 



