644 rROCEEDINGS OF THE NATIONAL MUSEUM. vol.50. 



Onh^ one small piece of the plastron present. Collected by J. B. 

 Hatcher. 



Locality. — Niobrara County (formerly a part of Converse County), 

 Wyoming. 



Horizon. — Lance formation, Upper Cretaceous or Lower Tertiary. 



The carapace of the type-specimen is about the size of the type of 

 Aspideretes hcecheri Hay from the same formation, but specifically 

 distinct as shown by differences in the sui-face sculpture and in the 

 arrangement of the costal and neural bones at the posterior end of 

 the carapace. 



The carapace is nearly as broad as long, and if measurements were 

 taken to the end of costal ribs the breadth would exceed the length. 

 The length must have been close to 300 mm., the width about 205 mm. 

 The shell is regularly convex from side to side. 



The lateral borders are somewhat sinuous, the hinder border 

 broadly truncated and without emargination. 



The sculpture of the carapace consists of a network of rounded 

 ridges inclosing irregularly rounded pits, usually without definite 

 arrangement, though there is a tendency toward the outer ends of 

 the median costais to dispose themselves in rows more or less parallel 

 to the borders of the shell. This feature, however, is not so apparent 

 as in A. foveatus (Leidy). The pits have concave bottoms, the walls 

 rising gradually as in A. foveatus, but this feature would at once 

 separate the present species from A. heecheri, which has the bottom 

 of the pits flat, the walls rising abruptly. The tops of the ridges are 

 always rounded in the present specimen, whereas in A. foveatus they 

 are usually flat. 



Tow^ard the free edge the pits increase slightly in size; they are 

 shallower and diminish in size tow^ard the middle, with here and 

 there small areas almost devoid of sculpture. Along the costal 

 sutures the pits have a tendency to elongate antero-posteriorly. This 

 Feature is not constant, and where present it is ahvays on the outer 

 halves of the costais. A line 10 mm. long extends across 6 pits, and 

 often 1 may be coimted. 



On the outer halves of the costais, posterior to the second, are seen 

 a number of prominent grooves that run from the border inward and 

 forward, diagonally across the longer axes of the costais. At first 

 sight they call to mind the welts observed on the costais of certain 

 species of tlie genus Plastomenus, but a close examination shows them 

 to be channel-like. The prominence of these channels is brought 

 about by the widening of the parallel ridges which inclose them. 

 Thin cross ridges divide the channel up into various size pits, which 

 are often subrectangular in form. 



The character of these channel-like markings is best shown in fig. 1, 

 plate 35. 



