1880.] NATURAL SCIENCES OF PHILADELPHIA. 389 



of this vein lying between the masseter and temporal masses, 

 behind the zvo-oma. The central tendon is thicker at the root of 

 the zygoma and the bone over the external auditory meatus than 

 any other localitj'' in the temporal fossa. 



The masseter has fine lajers, closely resembling those in the 

 dog. The deepest layer, namely', that one whose fibres occupy 

 the raraal fossa, has a much thicker aponeurosis than the other 

 la^-ers, the anterior portion of the first alone excepted. 



In the opossum, Bidelphys virginianus, the superficial fibi'es of 

 tlie temporal are everywhere thick. The aponeurosis is well de- 

 veloped. The supra-zygomatic slip is not distinct. The deep 

 portion of the muscle exhibits a white glistening tendon, which 

 does not, however, extend as far as the orbito-temporal septum. 

 The anterior portion of the muscle is made up as is usual by the 

 union of the deep and superficial portion. In addition to its 

 forming the slip passing down to the front of the base of the 

 coronoid, it sends a powerful bundle to the median side of the 

 coronoid, a thin movable laj'er of muscular tissue, which passes 

 in front of the coronoid, between the medio-coronoid and pre- 

 coronoid portions. 



The masseter is highly tendinous superficially. The tendency 

 to cleavage is not pronounced, and the continuity of the deep 

 fibres with the superficial fibres of the temporal is very noticeable. 

 In the squirrel, Sciurus hudsonicus, the superficial portion of 

 the temporal is less distinct than in man, and the supra-zygomatic 

 slip, while demonstrable, is not large. The superficial tendinous 

 layer of the masseter arises from a sijur on the maxilla below tUe 

 infra-orbital foramen. It passes, as is usual, downward and back- 

 ward toward the angle. This layer does not, as in most mammals, 

 form the entire superficies. A second layer arises from entire 

 inferior border of the zygoma, which appears to be lost upon the 

 foregoing about midway between the zygoma and the angle. Upon 

 turning this last layer downward, the third and last layer is seen, 

 which is continuous in the ordinary manner with the temporal 

 fibres. The ai-rangement of fibres on the median surface of the 

 mandible was not examined. 



In the North American porcupine, Erethizon dorsatus, the 

 masseter consists of a superficial set of fibres arising tendinously 

 from the malar bone, and passing downward and backward to the 

 angle of the mandible. It arises from the anterior three-fourths 



