PHYLOGENY OF SHELL OF TESTUDINATA 437 



the gastralia, none appears to be with certainty described. The 

 plastron appears to have been soHdlj^ united with the carapace 

 and no suture appears to separate the gastraha along the mid- 

 line. Under such conditions, how can it be assumed that there 

 were no hyoplastra, no mesoplastra, no hypoplastra, and no 

 xiphiplastra? Is it probable that this turtle, which in most 

 features resembles so closely other well-known forms, differed 

 from them all in having none of the ordinary plastral bones, 

 except the front ones, but instead of these a plastron composed 

 of distinct and Uttle modified gastralia? 



Jaekel finds that the gastralia of Triassochelys diverged as 

 they passed from the bridges toward the midline, and he gives 

 an explanation of the divergence. If, now, this plastron repre- 

 sents a primitive condition from which, through segregation 

 and consoHdation of the gastralia, were produced definitive 

 plastrals, how are we to explain the fact that in those turtles 

 which possess mesoplastrals the sutures between the plastral 

 bones converge as they are followed toward the midline? They 

 appear, therefore, not to have followed the sutures between the 

 gastralia, but to have struck across them at varying angles. 



There can be no doubt that Triassochelys is closely related 

 to Proganochelys. In this Triassic turtle Fraas (Jahresh. Ver. 

 vaterl. Naturk., vol. 55, 1899, p. 416, pis. VII and VIII) con- 

 vinced himself that there was present a pair of mesoplastrals, 

 greatly expanded at the outer ends. It seems that later Doc- 

 tor Jaekel (Placochelys placodonta, 1907, p. 59) succeeded in 

 shaking Fraas 's confidence in his determinations; but it appears 

 to the present writer that the probabilities are in favor of their 

 approximate correctness. How JaekeFs observations are to 

 be harmonized with the views here expressed the writer does 

 not at present comprehend. It may be noted in passing that 

 Doctor Jaekel was in error when he stated that Fraas beUeved 

 that there were in Proganochelys two pairs of mesoplastrals. 



Doctor Jaekel concluded that in Triassochelys the pectoral 

 scutes were missing. There appear to be no suffi.cient reasons 

 for this conclusion. The great scutes which bound the notches 

 for the fore legs are surely pectorals. In front of these scutes 



