1880.] NATURAL SCIENCES OF PHILADELPHIA. 391 



It will be seen that the plan of the muscles is the same as in 

 other mammals, but is remarkable for the muscles' subdivisions 

 remaining distinct from one another. In rodents having the large 

 infra-orbital foramen, the masseter muscle is described as having 

 a separate portion passing there through. Mivart, in his Elements 

 of Anatomy, page 309, says, in this connection: "In certain ro- 

 dents, e. g., Lagostomus and the Agouti, the masseter divides into 

 three portions, and traverses (that is, one of these portions tra- 

 verses) the singularlj" enlarged infra-orbital foramen." This is a 

 correct expression of the view usually taught. According to the 

 plan of description followed in this paper the masseter of Ere- 

 thizon in nowise differs from the muscles of the same name in 

 other mammals, except in the extent of development of the la3'er 

 to which the pre-foraminal fibres belong. I have had no oppor- 

 tunity of examining Lagostomus, but it is probable that the 

 masseters are much alike in all. The porcupine is further of 

 interest in the extent of encroachment of the muscular fibres 

 upon the orbital space. Both masseter and temporal appropriate 

 large surfaces. It is noteworth}^ in addition to find that the 

 l)ost-orbital process is here purely muscular in significance. It is, 

 indeed, imbedded in muscle. Notwithstanding its size, the process 

 has no septal significance in this rodent. 



In Goelogenys the temporal is thin in the temporal fossa but 

 thick and massive on posterior wall of the orbital space. The 

 superficial layer and supra-zygomatic slip are distinct. Raising 

 these two portions of the temporal from the temporal fossa no 

 muscular fibres are seen beneath. A distinct tendon becomes 

 visible, however, underlying the junction of the superficial and 

 supra-zygomatic portions In the orbital space the superficial 

 portion is exceedingly robust and extends medianly the entire 

 depth of the posterior wall. The temporal is inserted into the 

 lower jaw as follows : The superficial portion arising from the 

 temporal fossa, and the zygomatic portion are inserted through 

 the main tendon upon the apex of the coronoid process ; the orbital 

 portion upon the median side of the same tendon and the median 

 surface of the coronoid its entire length. 



Comparing the plan of this muscle to the others described it 

 may be said that the de6p part of the muscle is absent, unless the 

 greater bulk of the orbital portion is assigned to the deep part. 

 It has been generally found that the deep and superficial portions 



