34 



This carapace (fig. I) is a little broader thau long, and is only slightly conA-ex. 

 Seen from above, its anterior, posterior and lateral curves are flattened so as to make the 

 outline subquadrangular. The eight pairs of costals are entire and there are six neural 



Fig. 1. — Carapace of Trionyx Jovealiis, from Red Deer river ; a, outline of the transverse curve of its upper surface. 

 Five-eighths the natural size. NIX, Nuchal hone (restored) ; Nl, la, 2, &c., Neural bones ; Cl. 2. &c., Costal bones. 



bones of which the anterior one is divided. Neural 1 is broader than long with convex 

 sides, neural la is longer than broad, broadest near its posterior end and has concave 

 sides. Neurals 2, 3 and 4 are six-sided, neural .5 is oblong, and neural 6, lying for the 

 most part between the sixth costals, is shield-shaped, narrowing to a point behind. The 

 seventh costals are suturally united at their inner ends, where they develop a breadth 

 sufficient to separate the eighth costals from each other. These last are subtriangular in 

 shape, with three convex sides. The nuchal plate was evidently small, as the lateral 

 termination of the suture (c, in figure 1) between it and the first costal indi- 

 cates a side extension not far past a point in advance of the mid-length of the first costal ; 

 its lateral ends lie beneath the front border of the first costals and are not seen from 

 above. 



