PHYLOGENY OF SHELL OF TESTUDINATA 435 



the bones (fig. 1). In two cases the depression holding the 

 plate makes a hole through the shell, but this is only where they 

 He in the course of a sulcus where the bone is thin. These little 

 bones have a yellowish appearance, being thus somewhat dif- 

 ferent from those of the other specimens. Nevertheless, they 

 give the usual reaction with acid, and under the microscope they 

 show the haversian canals and the lacunae. 



What interpretation is to be put on these flakes of bone it is 

 difficult to say. It has appeared possible that they are repre- 

 sentatives of the mosaic of bony plates which are found between 

 the keels in Dermochelys. So far as the writer now sees, the 

 principal argument against this explanation is the irregularity 

 of distribution. It has been suggested by some scientific friends 

 that they are produced by parasites, but of this the writer has 

 seen no evidence. 



Still another shell of Chelys has been found in the collection 

 of the U. S. National Museum. This has the catalogue num- 

 ber 8602 and is recorded only as having come from Amazon 

 River. On this specimen there are no traces of either the plates 

 of bone which underlie the center of growth of the various horny 

 scutes, nor of those smaller plates which are scattered irregu- 

 larly over the shell. How to account for the condition the writer 

 does not know. Unless there is great variation in Chelys fim- 

 briata, this specimen must belong to another species than that 

 of the mounted one. It is possible that now and then an individ- 

 ual fails to reproduce such useless vestigial structures. At 

 least the writer believes that this case does not invalidate his 

 explanation of the presence of the bones found at the centers of 

 growth of the scutes. If now and then a cat should fail to have 

 the vestigial first upper molar, this would not prove that in 

 other cases this molar had not been inherited from the original 

 felids. 



Our study of the shells of Chelys has therefore resulted in 

 demonstrating the presence of epithecal bones which in the 

 writer's opinion, correspond to those of the median, first lateral, 

 and the marginal keels of the carapace and of the outer lateral 

 keels of the plastron of Dermochelys; besides numerous smaller 



JOURNAL OF MORPgOI^OQT, VOL. 36, NO. 3 



