PROCEEDINGS OF THE NATIONAL MUSEUM 



VOLj 77 



One of the outstanding structural features of T. sculpta seems to be 

 the great distal expansion of costals three and five and the consequent 

 narrowing of costals two and four. T. hilli (Cope) shows the costals 

 to be fairly uniform in width as they are in most extant species of 

 Pseudemys. So little is known of the costals of the other extinct 

 species of Trachemys that they do not permit of comparison being 



made with the present 

 specimen. 



Excepting peripheral 

 one which presents an 

 acute free border the 

 others are obtuse and 

 only slightly everted. 

 Over the bridge an ob- 

 tuse keel intervenes be- 

 tween the upper and 

 lower portions of the 

 shell. The free borders 

 of the posterior periph- 

 erals are acutely edged 

 and dis tinctly s c a 1 - 

 loped. 



The sulci are every- 

 where deeply im- 

 pressed. The form of 

 the various scutes may 

 be clearly determined 

 from Figure 1. The 

 table gives the dimen- 

 sions of the verte- 

 brals. 



The nuchal scute is long and narrow and raised into a strong ridge, 

 that projects slightly beyond the free edge. The sulci separating the 

 costal from the marginal scutes traverses the peripherals some distance 

 below the costo-peripheral sutures from the nuchal to the eleventh 

 peripheral, where it rises and crosses the midline on the posterior 

 border of the suprapygal, a condition occasionally found in the West 

 Indian Pseudemys rugosa and P. palustrus. In T. hilli (Cope) this 

 sulcus crosses the midline below the suture on the pygal, and the sulci 

 more nearly coincide with the costo-peripheral sutm-es, as they do in 

 most specimens of Pseudemys that have come under my notice. 



Figure 2.— Plastron of Trachemys sculpta Hay. Cat. 

 11839, U.S.N.M. Nearly One-third Natural Size 



No. 



