112 H. H. NEWMAN. 



observations point strongly to a periosteal origin of these struc- 

 tures, which means that^ they are in no sense dermal or the de- 

 scendants of the original dermal carapace. 



The testimony of comparative anatomy leads to the belief that 

 the nuchal, procaudal, pygal and marginal plates are the remnants 

 of a once more or less complete dermal carapace and that these 

 plates formed the cores of scutes that must have had a more or 

 less tubercular form. The keels of existing scutes represent these 

 tubercules. The testimony of the tail-trunk of Chelydra indi- 

 cates that there were originally seven primary rows of such scutes 

 and that less prominent rows of scutes occupied the interspaces. 

 These less prominent rows were gradually suppressed, first on the 

 carapace and then on the plastron, beginning in the middle and 

 proceeding laterally. Thus the first loss was the neuro-costal 

 rows, second the supra-marginals, third the interplastrals, and 

 fourth the inframarginals, \vhich to-day persists normally in many 

 primitive forms. 



Accompanying the suppression of rows occurred a reduction 

 in the number of scutes in the primary rows, and this reduction 

 took a general antero-posterior direction. At the same time the 

 rapid secondary growth of the neural spines and ribs caused the 

 suppression of the corresponding dermal plates, leaving only the 

 nuchal, procaudals, pygal and marginals in places where the in- 

 ternal skeletal portions failed to extend. Traces of the dermal 

 armor in the mid-neural region have been found, however, in 

 ToxocJiclys and Graptcmys. 



No correlation of abnormalities is to be expected in the neural 

 and costal regions, since the scutes and plates of this region are 

 entirely independent in origin, but in the marginal series, where 

 the plates and scutes retain nearly their original connections, the 

 correlation is perfect. In the procaudal region we find frequent 

 correlations, but, that the correlation is not a necessary one, is 

 shown by numerous uncorrelated abnormalities. 



A study of the color-markings of Graptentys and CJielydra 

 lends confirmation to all the above theories of the chelonian car- 

 apace and plastron, and at the same time serves to rationalize 

 the patterns themselves. 



