Cope. ] 568 (Jan. 18, 
and are a good deal longer than its height ; in the species named the height 
equals the other measurements. The pterygoid ala rises opposite the 
middle of the end of the glenoid surface, and the angle of its junction 
with the pyramidal process of the palatine is considerably in front of the 
middle of the trough of the posterior nares. Its edge posterior to this 
angle is shallowly grooved, The palatonareal border differs from that of 
any other species of the family known to me. It is acute in front, forming 
a Gothic arch, its apex being opposite the middle of the superior third true 
molar. In a young M. guyotianus, the only specimen of that species in 
which it is perfectly preserved, it is rounded, and extends to the posterior 
part of the second true molar. In an adult specimen, where the middle 
portion of the margin is lost, it extended at least as far forwards ; but its 
form is uncertain. The palate in the A. ryderanus is strongly concave 
throughout, 
The lachrymal bone has a different form from that of a A. guyotianus, 
more resembling that of A. latidens figured by Leidy. Its anterior superior 
angle is not produced, and its outline is a little deeper than long. The an- 
terior lateral prolongation of the frontal extends beyond it by nearly its 
width, and is wide, and terminates in an obtuse angle. The posterior edge 
of the nasals is broadly rounded, truncate at the middle, and is situated 
much in advance of the frontal foramina, The pariétal is in contact with 
the alisphenoid. The squamosal does not extend beyond the vertical line 
from the base of the paroccipital process. 
The infraorbital foramen is above the anterior edge of the third superior 
premolar, a position only seen elsewhere in the genus A. trifrons. The 
superior border of the orbit is concave and short as in A, guyotianus, and 
not straight and flat as in A. ¢rifrons. The frontal foramina are above 
their middle, and their distance apart goes 4.5 times into the interorbital 
width. There is a large postpariétal foramen on the pariétosquamosal su- 
ture, and a large postsquamosal immediately below it. This arrangement 
differs from that seen in the other species here described, where there are 
two or three postsquamosals well posterior to the postpariétal. Mastoid for- 
amen small. There are two palatine foramina on each side of the mouth, 
one opposite the posterior edge of the second premolar, and one opposite the 
posterior part of the fourth premolar. The anterior condyloid foramen is 
large. On one side isa small posterior condyloid, the only occurrence I 
have met with in the family, The foramen lacerum posterius is not divided 
into three foramina as in the A. guyotianus, but remains open as in the 
species of Hucrotaphus and Merycochawrus. It shows its nearer affinity. to 
the first named species, however, in its triradiate outline ; and in the three 
grooves of the side of the bulla, which correspond to two of the three fora- 
mina. The f. lacerum anterius is not large, and is oblong in shape. The 
ovale is rather small, and is entirely bounded on the inner side by the 
pterygoid ala of thesphenoid. The /. rotwndum is large and rather poste- 
rior. It is not bounded below by a transverse shoulder as is seen in the spe- 
cies of Merycocheerus, but is continued into a longitudinal groove, whose 
