148 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



In the Adapidae as a whole there are usually three mental foramina, which may be called the first 

 (I), second (II) and third (III) mental foramina respectively; typically the first foramen is beneath pi, 

 the second beneath ps, and the third beneath p4. In the Notharctinee the first foramen is occasionally 

 beneath the canine or between the canine and pi, and the second beneath p2. In some of the later Noth- 

 arctinse (A^. pugnax) there is a fourth foramen (IV) beneath nii, or in one extreme case beneath m-i. In 

 the Adapinse also there are usually three mental foramina, of somewhat variable position, but perhaps 

 more typically beneath pi, p3 and mi. Possibly the posterior foramen may represent No. IV of the 

 Notharctinse, while No. Ill may often be absent. 



Action of the Jaw Muscles 



From a study of the areas of origin and insertion and the consequent dii-ection of the jaw muscles 

 in the skull and jaw of A^. oshorni the following inferences appear highly probable. 



(1) The jaw was pulled forward and upward by the combined action of the masseters, pterygoidei 

 internus and externus; after the jaw was pulled forward these muscles, together with the tem- 

 porals, could exert strong pressure in the region of the canines and premolars. 



(2) The jaw was pulled upward and backward by the temporals; acting with the pterygoids this 

 would give a powerful vertical pressure on the molars. 



(3) The jaw was tilted toward the outer side by the combined action of both divisions of the masseter 

 and the temporal, possibly assisted by the pterygoids of the opposite side. 



(4) Oblique and lateral movements of the mandible were probably effected by alternate and selective 

 tension and relaxation of muscles on opposite sides. For example, the ental movement of the 

 right mandible in mastication (see page 139 above) was probably executed as follows: 



(a) tension of the left external pterygoid, the other muscles being relaxed, this pulling the left 

 condyle forward and inward, forcing the right condyle back against the postglenoid process 

 and swinging the whole jaw on the right condyle as a pivot toward the right; the right 

 temporal and the right masseter co-operate by slightly tilting the right ramus outward, 

 the right condyle being the pivot; this slightly depresses the left condyle, and brings the 

 outer cusps (protoconid, hypoconid) of the right lower cheek teeth nearly opposite the 

 outer cusps (para- and mesostyles) of the right upper teeth; 



(b) a maximum tension of the right temporal, right masseter and right internal pterygoid, 

 producing great vertical pressure on the cheek teeth; 



(c) increasing tension of the left temporal, left masseter and left internal pterygoid, relaxation 



of the left external pterygoid, all tending to pull the whole jaw toward the left side and 

 causing the lower teeth to sweep across the upper teeth from right to left, that is from the 

 outer side inward (ental) ; 



(d) relaxation of the temporals, masseters and internal pterygoids of both sides, combined with 



(e) tension of the digastric of both sides, lowering the jaw, and tension of the right external 

 pterygoid pulling the right condyle forward and inward and swinging the right ramus 

 toward the left. 



In Adapis the motion of the jaw was probably more orthal and less ental. The huge development 

 of the masseters, as indicated by the great size of the malar, and of the internal pterygoids, as indicated 

 by the expansion of the pterygoid fossae of the jaw, suggests that great force could be exerted by vertical 

 pressure of the sharp-edged canines, incisors and premolars; the sharp protolophs and protolophids also 

 would be effective in chiefly vertical but slightly transverse movements of the mandible. 



