268 O. P. Hay — Fossil Turtles in the Marsh Collection. 



plates, instead of eight pairs ; twelve pairs of peripheral bones, 

 instead of eleven pairs ; six vertebral scutes, instead of five ; five 

 pairs of costal scutes, instead of four pairs ; and twelve pairs 

 of marginal scutes, instead of eleven pairs. There is no doubt 

 regarding the presence of any of these extra bones and scutes, 

 since all the sutures and sulci are very distinct. 



A comparison of this carapace with Leidy's figure of Emys 

 wyomingensis shows that the whole anterior portion agrees 

 with that figure, only such deviations as might be expected in 

 two individuals of the same species being present. The pos- 

 terior third, however, leads one into difficulties. 



Figure 3. — Chrysemys ivyomingensis. 

 possessing supernumerary structures. 



Diagram of carapace of individual 



The normal number of costal plates is, of course, eight pairs. 

 In some of the living species of North American Trionychidse 

 the costals are reduced to seven pairs. Dr. Boulenger has 

 stated that in some fossil marine turtles there are nine or ten 

 pairs of costals, but he has not mentioned the species. Such 

 cases may occur, but it is doubtful that they are normal forms. 

 Mr. L. M. Lambe (Contr. Canad. Palaeont., iii, 1902, p. 42, 

 H^. 7) has described as a new genus and species Neurankylus 



