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MONOGRAPHS OF NOBTO AMERICAN RODENTIA. 



The temporal muscle is of moilernte bulk, the fossa being comparalively 

 shallow. The masseteric and pterygoid masses are of great size. The mas- 

 seter forms a bulging mass on the outside of the jaw, completely filling the 

 great fossa formed by the outward twist of the angle of the juw, and defined 

 in front by an oblique ridge already described in speaking of the bone. A 

 special stout tendon arises from the zygomatic process of the maxillary, just 

 below the anteorbital foramen, which latter opening, notwithstanding its small 

 size and apparently inconvenient relations, transmits a small fascicle of the 

 niasseter along with the superior maxillary nerve. 



The muscles acting upon the hyoid bone, both from the thorax and from 

 the jaw, are well developed. A pair of stout fusiform muscles connect tiie 

 liyoid with the back of the skull. I find no trace of direct muscular connec- 

 tion between the hyoid and the scapula (omo-hyoid). Sterno-mastuid and 

 cleido-mastoid are well developed, and distinct from each other for nearly, if 

 not quite, all of their extent, though their thoracic insertions are very near 

 together. 



The diaphragm is very thin, even its most muscular portions, and a large 

 port of it is simply membranous. There is a large, well-defined, central 

 "tendon", as broad as the muscular portion on either hand. This is of oval 

 shape ill most of its extent, but with two posterior prolongations, one on each 

 side, separated by the fleshy "pillars" which arise from the vert«brjB, and pro- 

 ceed to embrace the oesophageal orifice. Muscular fibres are scarcely or not 

 developed laterally behind, where simple membrane may be traced to the 

 insertion of the organ along the floating rib. The radiating muscular portion 

 of the diaphragm, then, is a single set of fibres arranged in fan-shape around 

 the anterior oval portion of the central tendon ; these fibres are continuous 

 on the median line in front. The aortic opening, as usual, is close to the ver- 

 tebrae ; the oesophageal aperture is removed from the spinal column by the 

 whole length of the muscular pillars. From the middle line of the diaphragm 

 depends a broad peritoneal fold, suspending the liver, to which, more posteri- 

 orly, it is closely adapted. 



E. — HISTORY AND HABITS OF THE SPECIES. 



To render the account of this remarkably interesting animal more com- 

 plete, I shall, in tracing its history, include some notices of its habits. I have 

 already presented those considerations which bear upon the history of the 

 genus and family. 



