IO4 H. H. NEWMAN. 



V. FURTHER EVIDENCE OF THE FORMER EXISTENCE OF A 



DERMAL CARAPACE IN CHELONIA, AS DERIVED FROM 



SPECIMENS OF GRAPTEMYS. 



The position just taken rests on the assumption that the cara- 

 pace and plastron were at one time continuous with that portion 

 of the trunk just posterior to them and that the carapace and 

 plastron have undergone a gradual process of specialization that 

 has caused them to depart widely from ancestral conditions. The 

 tail-trunk would then preserve to a greater or less extent its 

 original character. 



On this assumption, then, there once existed a complete row 

 of dermal tubercular ossicles overlying the vertebrae. That 

 certain ancient forms did actually possess these ossicles was shown 

 by Hay in the case of Toxochelys serrifer, but the question arises 

 whether or not we have sufficient evidence that Toxoclielys repre- 

 sents the ancestral condition of our modern forms. 



For a long time I looked for definite traces, other than the 

 keels, of such ossicles as are seen in Toxochelys, but met with 

 no success until I had nearly completed the present paper. Then 

 by merest chance I stumbled on the evidence needed to clinch 

 the argument. 



I had kept alive a few specimens of Graptemys in a small 

 aquarium, but one by one they sickened and died, with one ex- 

 ception. Their death was doubtless due to the fact that this 

 species is highly specialized in its diet, feeding exclusively on a 

 species of viviparous gastropod that is abundant in Lake Max- 

 inkuckee. They never learn to use other food, and, in lack of 

 their special diet, starve themselves to death. 



The surviving specimen was a nearly adult female that had 

 been kept on account of its many peculiarities. After eleven 

 months of captivity it was killed and examined for plate abnor- 

 malities. This examination revealed the presence of several 

 small, loose, ossicles that were inlaid, as it were, in the bone of 

 the neural plates and were situated beneath the keels of the sec- 

 ond, third, fourth and fifth neural scales, /. e., exactly in the 

 positions occupied by the ossicles found in Toxochelys. The 

 largest of these ossicles (see Fig. 5) was situated beneath the 

 keel of the third scute and extended partially under the anterior 



