TRIONYCHID^:. 



4*7 



Fauna from the Judith River and Great Lignite Formations of Nebraska," publisht as cited 

 above. These parts, now preserved in the Academy of Natural Science at Philadelphia, 

 must be regarded as the types of the species. A fragment of the right last costal plate, found 

 near Long Lake, was identified, with some doubt, as belonging to the same species, and 

 furnisht fig. 3 of Leidy's plate above cited. This specimen is now in the National Museum at 

 Washington and bears the catalog number 990. 



As regards the type specimens, there seems to be no certainty that the costal plate and the 

 fragments of plastron were found associated. We must, however, for the present, assume 

 that they belong to the same species. Should future discoveries prove that two species are 

 involved, we shall have to take the costal plate as the type. 



This costal is 22 mm. wide, 5 mm. thick at the sutural margins, and 7 mm. thick thru 

 the rib. The ornamentation consists of pits of varying size and shape separated by ridges 



whose summits are usually flat. There 

 are, in general, 5 pits in a line 10 mm. 

 long. The pits are, as stated by Leidy, 

 smaller at the proximal end of the costal 

 than further outward; but this increase 

 in size is due to the narrowing of the 

 separating walls, not to a reduction in 

 the number of pits in a given line. The 

 pits are concave, and give the impression 

 of having been scoopt by an engraver 

 in a level surface. Near the sutural mar- 

 gins of the costal the pits are almost 

 wholly wanting and the surface is level 

 and smooth. 



The fragments of the plastron of the 

 type are 8 mm. thick. The lower surface 

 is covered with shortvermiculated ridges, 

 some of which inosculate with the neigh- 

 boring ridges while others do not. 

 Scattered among these are small dot-like 

 elevations. 



Mr. L. M. Lambe, of the Geological 

 Survey of Canada, described, as cited in 

 the synonymy above, a nearly complete 

 carapace and some plastral bones which 

 he refers to this species. These he 

 secured in the Belly River deposits, on 



the Red Deer River, in Alberta, British America. The nuchal bone is missing from the cara- 

 pace. The length of the carapace (plate 89, fig. i; text-fig. 643) as found, is 163 mm. The 

 width of the nuchal has been estimated by Lambe as 5 mm. but it is probable that the 

 width fore and aft was considerably greater. The maximum width of the carapace is 214 mm. 



The rear is broadly rounded, and the upper surface is moderately 

 convex. There are present a preneural, 6 neurals, and 8 pairs of 

 costals. The genus Aspideretes is therefore indicated. 



The preneural is short, being 18 mm. wide and 12 mm. long. 

 The 4 anterior neurals are broader behind than in front; the 

 fifth is a parallelogram; the sixth is broad in front and pointed 

 behind. The dimensions of the neurals are given in the table. 

 The eighth costals are small, almost vestigial, as in some 

 species of Platypeltis, the lateral extent of one being about 22 

 mm.; of the other, 20 mm. Each lies in a notch in the costal in 

 front of it, and they do not come into contact at the midline, the 

 latter condition being probably an individual peculiarity. The sculpture resembles closely that 

 of Leidy's type. It is thus described by Lambe: 



FIG. 643. Aspideretes foveatus. Carapace. 

 Figure by L. M. Lambe. 



a, line showing curvature of shell from side to side; c.f.l, c.p.8, 

 the costal bones; pren, preneural; n. I, n. 6, the neurals; nu. p, 

 nuchal bone. 



