ABNORMALITIES IN CHELONIA. IO3 



jectured from an examination of the directions of encroachment 

 of the principal rows upon the secondary rows. On this basis 

 one might give the order of loss as follows : first, neuro-costals, 

 which on account of inward encroachment of the costals and the 

 outward encroachment of the neurals, would receive the severest 

 pressure ; second, the supramarginals located between the in- 

 wardly-encroaching marginals and the outwardly-encroaching 

 costals ; third, the interplastrals, between the two inwardly- 

 encroaching plastrals, but occasionally able to escape pressure by 

 occupying angles between four plastrals ; fourth and last, the 

 inframarginals, which were subject to pressure only on one side 

 that of the marginals and were in addition required in more 

 primitive forms with small plastrons to help bridge the space 

 between the carapace and plastron. With the great increase in 

 the size of the plastron which has taken place in higher forms, 

 the space allotted to inframarginals became more and more con- 

 tracted until they were crowded out entirely. 



Does this order of loss correspond to any facts in nature ? The 

 same conclusion as to order of loss, I believe would be reached 

 if we based our results on the prevalence, scarcity or absence of 

 these rows as normal structures in existing species. No trace of 

 neuro-costal scutes are found. Only one species, Macrochelys 

 tennnincki, possesses supramarginals. Interplastrals occur as iso- 

 lated median ventral scutes in several families. Inframarginals 

 occur normally in all the more primitive families and as axillaries 

 and inguinals in all but the most specialized terrestrial forms. 



This condition as derived from the tail-trunk of CJielydra differs 

 rather radically from the ancestral condition derived from Der- 

 mochelys, in which there are twelve rows of equal rank. I am 

 inclined to believe that DernwcJielys is an extremely aberrant type 

 with only a most distant connection with the phylogenetic line of 

 Chelonia. The twelve keels of Dermochelys are comparable, I 

 believe, to the seven keels of modern forms, and the irregular rows 

 of plates and scutes between the keels are comparable to the 

 secondary rows of scutes seen in the tail-trunk of CJiclydra and 

 represented in the carapace and plastron by inframarginals, supra- 

 marginals, interplastrals and neuro-caudals. 



