IO6 H. H. NEWMAN. 



of Graptemys geograpJiica, and that the most prominent tubercle 

 is that on the third scute, a fact that is of interest in view of the 

 larger size of the vestigial ossicle on that scute. 



The adult of Graptemys psctidogcographica retains decided 

 traces of these tubercles throughout life and their location is 

 marked by dark blots of pigment that in later life form the only 

 prominent color-markings of the animal. The discovery of ves- 

 tigial ossicles in Graptemys geographica led me to investigate a 

 very large female specimen of Graptemys psendogeographica, that 

 has been in confinement, and, like its relative, starving for many 

 months. Immediately beneath the dark blots of pigment on the 

 neural scutes there are on this specimen thin, scale-like discs of 

 bone with a looser and less dense texture than the underlying 

 bony plates. I have been able to examine but one adult spec- 

 imen of Graptemys psciidogeograpliica, but this one appears to be 

 perfectly normal in every other respect. It is to be noted that 

 the vestigial plates in the last-mentioned specimen occupy exactly 

 similar positions, with respect to scutes and plates, as do the ves- 

 tigial ossicles in the anomalous specimen of Graptemys geo- 

 grapliica, described and pictured above (Fig. 5). The probable 

 explanation of both these conditions is that long-continued star- 

 vation has brought about a resorption of the portions that 

 united these ossicles with the underlying neurals. That bone is 

 resorbed either by normal processes or as the result of patholog- 

 ical conditions I have observed in several cases where holes have 

 been eaten entirely through the bone of the carapace, as the 

 result of starvation. Only a thin cap covering the top of the 

 tubercle is of dermal origin, the main portion of the prominence 

 being merely an outgrowth of the periosteum of the neural 

 process. Examinations of developmental stages have revealed 

 no discontinuity between the cap and the rest of the keel, but the 

 microscope reveals the difference in histological structure be- 

 tween the cap and the underlying bony plate. 



That tubercular ossicles existed over the neural processes of 

 other ancient reptiles is shown by the fossil Stegosaurus. Here 

 the dermal processes are very large and prominent and are much 

 fewer in number than the neural processes that underlie them. This 

 points to the entire independence of dermal bones and the vertebrae, 

 and hence to the non-segmental character of the dermal plates. 



