ABNORMALITIES IN CHELONIA. 73 



nous skeleton, is not certain. Haycraft ('99) maintains the lat- 

 ter view, but his paper is far from convincing. 



As to the remaining plates of the carapace nuchal, pro- 

 caudals, pygal and marginals there is no difference of opinion. 

 All agree that they are of true dermal origin. 



Thus it would seem that the plates of the carapace have a dual 

 origin the neurals and costals being periosteal ossifications 

 while the nuchal, procaudals, pygal and marginals are dermal 

 ossifications. 



The carapace, then, as it exists to-day is not a simple struc- 

 ture but consists of a complex of at least two independent systems 

 of bones. 



Accepting the evidence of embryology as to the origin of the 

 neural and costal plates, it remains to determine whether the 

 dermal ossifications are, as Goette believes, mere supplementary 

 structures that have come in to supply the deficiencies of the 

 periosteal system, or are remnants of a once more or less complete 

 dermal carapace that has in large measure been rendered super- 

 fluous by the broadening-out of the ribs and neural processes. 

 The latter view would involve the former existence of complete 

 rows of dermal bones overlying the vertebrae and ribs. Embry- 

 ological evidence seems contrary to this view, as no dermal ossi- 

 fications are found in the costal or mid-neural regions. It is 

 possible that we may in this case overestimate the evidence of 

 embryology as a guide to phytogeny. The great antiquity of 

 the chelonian carapace is undoubted and in highly specialized 

 structures that have attained a marked morphological fixity we 

 should not be surprised to find great condensation in develop- 

 ment, so that two structures formerly independent in origin 

 such as dermal and periosteal plates may originate simulta- 

 neously so as to form only one inseparable structure. It seems 

 quite plausible, then, that the rapid secondary broadening of 

 ribs and neural processes has crowded out or appropriated the 

 primordia that formerly went to form the dermal carapace and 

 that only in places where the ribs and neural processes fail 

 to reach the dermis do the true dermal bones have a chance 

 to appear. 



1 he fact that the nuchal plate appears before the ribs and 



