TRIONYCHIDtE. 



501 



half. Toward the midline the hypoplastron becomes much wider than the hvoplastron. 

 The hypoplastron sends inward a finger-like process, which no doubt joined a similar process 

 from the bone of the opposite side. The median border of both bones was very obliquely- 

 beveled. The greatest thickness of the hypoplastron is 12 mm. The xiphiplastron present 

 appears to belong to the right side. It has a central, thick portion, sculptured, oval, 25 mm. 

 wide, and about 40 mm. long. This passes by a bevel into the thinner portion on all sides. The 

 thinner part extended forward into digitations joining the hypoplastron, and posteriorly into 

 others which connected with the corresponding bone of the opposite side. It is evident that 

 there were large fontanels between the right and left halves of the plastron. 



The sculpture (plate 94, figs. 1-3) consists of a network of ridges which inclose pits ot 

 rather regular form and size. The pits are of somewhat greater diameter than the ridges sur- 

 rounding them. On the neurals there are about 5 pits in 12 mm., while on the hindermost 

 costals there are only about 3 in the same distance. The pits increase somewhat in size from the 

 proximal toward the distal ends of the costals. There is no tendency, near the free margins 

 of the costals, toward either the disappearance of the ridges or toward their breaking up into 

 tubercles. There are no welts on the surface such as we find in species of Plastomenus. 



The sculpture of the plastron is greatly like that of the carapace. The sculptured part ot 

 the hvoplastron is nowhere more than 30 mm. wide, while that of the hypoplastron is 53 mm. 



Aspideretes singularis sp. nov. 

 Figs. 656-662. 



The type of this species is a nearly complete specimen which was collected for Prof. 

 E. D. Cope, by Mr. David Baldwin, in 1883, from the Torrejon beds of Chaco Canyon, San 

 |uan County, New Mexico. It furnishes the oldest-known trionychid skull that is accom- 

 panied by the shell. 

 Conchochelys admira- 

 bilis, of the underlying 

 Puerco beds, furnishes 

 an older skull, but the 

 shell is entirely un- 

 known. 



The carapace (fig. 

 656) must have been 

 rather convex, but the 

 convexity was probably 

 increast during fossili- 

 zation. The length is 

 350 mm. The width of 

 the disk could not have 

 exceeded 280 mm. Be- 

 yond this the rib-ends 

 extend 50 mm. on each 

 side. Therefore the 

 whole width of the 

 carapace was about 

 380 mm. It is com- 

 posed of the nuchal, a 

 preneural, 7 neurals 

 and 8 pairs of costals. 

 The nuchal extends 

 from side to side 190 



mm.; fore and aft, 37 mm. The outer ends of the bone are smooth, the central portion is 

 pitted. The free borders of the carapace are beveled and smooth, but the general surface 

 is carved into ridges and inclosed pits. On the neurals and adjoining parts of the costals 

 the pits are nearly circular and there are 3 or 4 of them in a line 20 mm. long. Distally the 



FlG. 657. Aspideretes singularis. Plastron. 



Xi. 



