TRIONYCHID. 50! 
half. Toward the midline the hypoplastron becomes much wider than the hyoplastron. 
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 of 
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 of 
the hyoplastron is nowhere more than 30 mm. wide, while that of the hypoplastron 1s 53 mm. 
Aspideretes singularis sp. nov. 
Figs. 656-662. 
The type of this species 1s 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 
Juan County, New Mexico. It furnishes the oldest-known trionychid skull that 1s accom- 
: panied by the shell. 
Conchochelys admtra- 
bilts, 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 
350mm. 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. 
Fic. 657.—Asprderetes stngularts. Plastron. » i, f The nuchal extends 
+ rom 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 ina line 20 mm. long. Distally the 
Ea lrg 
