NO. 2 A STUDY OF MENISCOTHERIUM — GAZIN 2$ 



the temporal fossa, and terminates in the lambdoidal crest just above 

 the mastoid portion. 



The Phenacodus skull in lateral view shows a relatively greater 

 depth of the snout and a more distinctly convex longitudinal profile. 

 The premaxilla is more slender, and the portion carrying the incisors 

 projects forward, more shelf like than in Meniscotherium. The 

 anterior portion of the zygoma joins the rostrum much as in Menisco- 

 therium, but the orbit is slightly more posterior with its anterior 

 margin approximately above the middle of the second molar and the 

 larger infraorbital foramen opens anteriorly a little farther back, 

 more nearly above the first molar. The lachrymal foramen is 

 similarly placed just within the orbital rim partially concealed by the 

 somewhat larger lachrymal tubercle. As well as being more subdued, 

 the postorbital processes are more posteriorly placed with respect to 

 the tooth row, and as previously noted the postorbital constriction is 

 distinctly farther back. It is particularly noticeable that the zygo- 

 matic process of the squamosal in Phenacodus is relatively shorter 

 and much more slender than in Meniscotherium, lacking the dorsal 

 flare above and anterior to the region of the glenoid surface. In 

 small Hyopsodus the orbit is located with respect to the dentition 

 about as in Meniscotherium, but the infraorbital foramen is a little 

 farther forward and perhaps relatively closer to the tooth row. More- 

 over, the jugal seems relatively deep for so small a form. 



Within the orbital fossa of Meniscotherium the orbital plate of the 

 maxilla presents a relatively large, broadly concave superior surface 

 exhibiting the large posterior opening of the infraorbital canal at its 

 anterior extremity. The sphenopalatine foramen opens near its pos- 

 terior margin but faces laterally in the adjacent ascending plate of the 

 palatine, very near the notch in the posterior margin of the hori- 

 zontal plate of the palatine. Immediately below but actually con- 

 fluent with the larger aperture of the sphenopalatine foramen is the 

 more dorsally facing posterior opening of the posterior palatine 

 foramen. Farther back in the fossa the optic foramen, if correctly 

 identified, is located slightly higher and well in advance of the 

 sphenoidal fissure, apparently near the anterior margin of the orbi- 

 tosphenoid. Posterolateral to the sphenoidal fissure and lower but 

 not far removed is the anterior opening of the alisphenoid canal 

 which, as determined from damaged specimens, is confluent in its 

 anterior portion with the foramen rotundum. 



The orbital plate of the maxilla in Phenacodus is more elongate 

 and relatively narrower than in Meniscotherium. The posterior pala- 



