170 



dates. The interior of the shell is occupied by a greenish-gray sandstone, 

 from which I obtained a pair of sacral vertebrae. 



The outline of the shell in its perfect condition was ovoid as in ordinary 

 Emydes; narrower and more elevated in front, wider and more depressed 

 behind. The fore part and sides of the carapace are uniformly convex, but 

 the hind part appears to have had the margin somewhat recurved. Over the 

 position of the vertebral scute areas the surface is flat and even and without 

 a carina. The plastron is flat, but its bridges turn from their commencement 

 upward and outward to the border of the carapace, which is elevated 2\ 

 inches above the level. The highest part of the shell is nearly 6 inches 

 above the level of the plastron. The bones of the shell, especially those of 

 the carapace, appear co-ossified, but not so completely as in Baena arenosa, 

 for those of the plastron can be distinctly traced. 



The intermediate vertebral scute areas have nearly the form and propor- 

 tions of those of Baena arenosa, and are rather longer than wide. The costal 

 scutes widened more outwardly than in that turtle, indicating a proportion- 

 ately greater degree of prominence of the shell. 



The lateral marginal scute areas are much like those in Emydes, but the 

 groove defining them from the costal scute areas exhibits an unusually undu- 

 lating course, not angular but serpentine or waving. 



The number and relative position of the scute areas of the plastron and its 

 bridges are the same as in Baena, but the median sternal groove defining 

 them on the two sides is remarkable for its irregular serpentine course, 

 repeatedly crossing the also somewhat irregular course of the median suture 

 of the plastron. 



The sutures of the plastron being visible, they reveal to us an unexpected 

 peculiarity, the existence or absence of which cannot be determined in the 

 shells of Baena arenosa from the total obliteration of the sutures. 



The peculiarity in the plastron of Chisternon, to which the genus owes its 

 name, is the presence of a large triangular bone, added to those which usually 

 exist iu turtles, on each side of the shell. This intercalated or mesosternal 

 bone commences at the center of the plastron and gradually widens outwardly 

 to where it conjoins the marginal plates of the carapace at the intermediate 

 half of the sternal bridge. The four sutures defining the mesosternal plates 

 from those in front and behind cross the plastron obliquely. A similar bone 



