GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



165 



these forms remains as a simple ring, incomplete above, fastened posteriorly to the periotic, in front of 

 the carotid foramen described below, and anteriorly to the squamosal behind the entoglenoid process. 

 The true tympanic cavity lies between the tympanic ring, the auditory prominence, or cochlea, and the 

 sinus hypotympanicus. The tympanic cavity is continued forward to the Eustachian foramen (ostium 

 tubae). Above the tympanic cavity and above the auditory ossicles a membranous diverticulum in the 

 periotic roof of the tympanic cavity near the squamosal is the recessus epitympanicus. 



The cochlea, or promontory, bears on its outer surface a long bony canal, for the main internal carotid 

 artery, which enters at the posteroexternal angle of the bulla in front of the stylomastoid foramen, runs 

 forward and inward over the cochlea to the anterior end of the hypotympanic sinus, medial to the Eu- 

 stachian foramen; here it pierces the back part of the basisphenoid and tunneling this bone emerges 

 beneath the cerebrum on either side of the sella turcica. Soon after entering the hypotympanic cavity 

 it gives off a stout branch, the stapedial canal which pierces the stapes and enters the periotic. 



Both Adapts, as described by Major and by Stehlin, and Nothardus, as shown in the type of 

 A^. osborni, conform in every detail to the foregoing description of the tympanic region, except that in 

 N. osborni the very delicate tympanic ring is not preserved. By comparison with Adapts, Lemur 

 and Propithecus, however, there can be no doubt whatever that the position of the ring was substantially 

 the same as it is in those genera: namely, that it was attached posteriorly to the junction of the 

 post-tympanic process of the squamosal with the outer wall of the bony carotid canal, at the postero- 

 external angle of the bulla, anteriorly to the posterior wall of the entoglenoid region of the squamosal, 

 internal to the postglenoid foramen. Nor can there be any doubt that the bulla covered the tympanic 

 ring. Direct evidence in favor of this view is given by a specimen of N. venticohis (Amer. Mus. No. 14655) ; 

 the surface of the bulla in this specimen is completely preserved and extends laterally to the auditory 

 meatus as it does in Lemur; as there is no possibility that the tympanic annulus was external to this edge 

 it must have been inside the bulla. Indirect evidence for the same view is that in the type of A^. osborni 

 the broken edges of the bulla show the contact of the expanded portion of the bulla on the 

 inner side of the entoglenoid region as in Lemur and Propithecus and that on account of the close 

 agreement with Lemur in the portion of the bulla that is preserved, the roof of the bulla must have 

 extended from this point of contact with the entoglenoid to the entrance of the carotid canal, and must 

 therefore have covered the ring from below. But it is impossible to convey briefly in words the full 

 force of the evidence for this immediate deduction from the facts, which appears inevitable after repeated 

 study of the specimens. Additional indirect evidence that the ring in Nothardus was not outside the 

 bulla results from a comparison with the tympanic region of the South American monkeys. For in 

 these (Plate XLIX) the great widening of the brain-case, as compared with that of Nothardtis, has evi- 

 dently caused a relative displacement outivard of the bony auditory meatus and of the attached ring, 

 and inward of the bulla itself: the ring being thus drawn to the outer side of the bulla, and increasing 

 greatly in size, yet retains its ancient contact with the entoglenoid region, while the bulla itself, not rela- 

 tively reduced in size, has lost that contact; meanwhile the opening of the carotid canal retains its old 

 place behind the ring, but is now also internal to it and presents inward or inward and backward rather 

 than outward and backward ; moreover the wide ring has now gained contact with the mastoid, while in 

 the Eocene Notharctus and Adapis it was separated from it by the carotid canal. In brief, comparison 

 with the conditions in the South American monkeys emphasizes the fact that in both the Eocene lemuroid 

 genera named above the relations of the parts of the tympanic region were fundamentally the same as in 

 modern Lemuridae and Indrisidse. (Plate XLIX.) 



