Cope.l OVO [Jan . is, 



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 parietal 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 superioi 

 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. trifrons. The frontal foramina are above 

 their middle, and their distance apart goes 4.5 times into the interorbital 

 width. There is a large postparietal foramen on the parietosquamosal 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 postparietal. 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 is a 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 Eucrotaphus and Merycoclmrus. 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 '/. 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 the sphenoid. The /. rotundum is large and rather poste- 

 rior. It is not bounded below by a transverse shoulder as is seen in the spe- 

 cies of Merycochcerus, but is continued into a longitudinal groove, whose 



