166 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



Along with this fundamental identity of plan there are minor differences in detail. The bullae of 

 N. venticolus were but httle inflated as compared with those of Adapis; those of N. crassus were probably 

 wider and more inflated than in A^. venticolus; the conditions in osborni were intermediate. There 

 were also considerable differences in shape and proportion of the bullse in different forms of Adapis as 

 figured by Stehlin, some being very wide {Adapis parisiensis var. Schlosseri, Stehlin, 1912, p. 1208, fig. 

 261, p. 1168, fig. 244), others narrower (Adapis viagnus, p. 1255, fig.) and smaller {A. 7nagnus var. leen- 

 hard.ti, p. 1279, fig. 287). The latter, which is one of the older varieties of Adapis, approaches Nothardus 

 in the general appearance of the auditory region. There were also difTerences in the width of the contact 

 between the entoglenoid ridge and the bulla, which was very wide in A. parisiensis Schlosseri (p. 1168) 

 and narrow in A. viagnus leenhardti. In both A. parisiensis and A. magnus the tympanic ring was 

 more or less enwrapped in its ventral half by a thin sheet of bone, which seems to represent an infolded 

 part of the wall of the hypotympanic sinus, where it has grown over the tympanic ring.^ As this portion 

 of the bulla is broken away in the type of Notharctus osborni we cannot be sure that a similar structure 

 was absent in the Notharctinse. 



In the type of Notharctus osborni the foramen ovale opens more in front of and less to the outer side 

 of the Eustachian foramen than it does in Adapis and the lemurs (cf. Stehlin, 1912, p. 1205) and the 

 bone between these foramina was not produced downward into a delicate septum. Leading from the 

 foramen ovale is a groove for the ramus mandibularis of the fifth nerve, running outward and forward 

 and opening on the outer side of the pterygoid wing of the alisphenoid; below the groove in question 

 there was a large foramen pterygospinosum near the junction of the pterygoid wing with the entoglenoid 

 region of the squamosal; so that, as in Adapis, the foramen ovale may be seen from the outer side through 

 the foramen pterygospinosum. According to Stehlin (p. 1205) this foramen probably transmitted a 

 branch of the internal maxillary artery to the internal pterygoid muscle as in Lemur. 



The anterointernal extensions of the bullse are closely appressed to the hinder edges of the basi- 

 sphenoid and true pterygoids and there are no foramina lacera media, the carotids entering the brain- 

 case by another route described above (p. 178). 



Basisphenoid 

 Text Fig. .58 



. The middle of the lower surface of the basisphenoid bears a convex ridge which gradually bifurcates 

 both posteriorly and anteriorly. The posterior forks are continued onto the basioccipital along the internal 

 wall of the bullse; the anterior forks run forward and outward toward the palatines. In Adapis there 

 is some variation in the form of the median basicranial ridge, which sometimes extends from the vomer 

 to the foramen magnum but never shows the doubly bifurcating pattern which is characteristic of 

 Notharctus. 



The VOMER is not preserved. 



The posterior part of the presphenoid is preserved, but is covered by the vertical plates of the pala- 

 tines. 



^ Dr. Stehlin (1912, p. 1294) refers to "die Verknocherung der Annulus membran" and states (p. 1213) "das [in Adapis] sich das 

 Septum an der vordern Aussenwand der Bulla S2 [of Lemur] nach hinten zu bedeutend vergrossert hat durch totale Verknocherung der 

 zum Annulus hintiber gespannten Membran." Though a little uncertain as to Dr. Stehlin's meaning, I infer from the illustrations that 

 the ossified membrane in question is not the true membrana tympani but a fold of the hypotympanic region of the bulla surrounding 

 the pneumatic foramen. 



