574 U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



process forms the outer margin of an elongated foramen, the anterior extremity notching widely 

 the frontal bone. 



The skull of Lepus cuniculus forms a sort of connexion between these two groups. In this the 

 post-orbital processes are widely divergent, and bound notches ; the muzzle is very slender in 

 all diameters, and much elongated ; the width greater than the height. The nasal bones are 

 very long. Compared with the skull of L. americanus, of nearly the same length, the muzzle 

 is little more than three-fourths as high just anterior to the molars ; the nasal bones one-fifth 

 longer. The incisors are larger. 



The next section embraces L. sylvaticus, bachmani, and artemisiae. Here the width of the 

 skull is again almost half the length. The cranial portion of the skull is short, in comparison 

 with the facial ; its upper outline is very convex, especially in the portion occupied by the parietal 

 bones; the posterior projection of the occipital bone is much below the le^el it exhibits in the 

 other groups. The muzzle anterior to the molars is wider than high. The post-orbital 

 process of the frontal bone is linear, and, projecting backwards, becomes anchylosed by its tip and 

 terminal portion of the inner edge with the cranium (the suture visible, however) ; there is left 

 a narrow, ovate foramen at the basal half or third of the process, the space enclosed not being 

 quite filled up. Lepus auduboni differs only in having a very narrow interval between the 

 posterior extremity of the post-orbital process, leaving a narrow notch instead of a foramen. 



Lepus trowbridgii differs still more in having the post-orbital process shorter and more diver 

 gent, so that the notch is more conspicuous, somewhat as in the first form mentioned. 



The remaining section, embracing L. aquaticus SLuApalustris, is most remarkable of all. Here 

 the skull and incisors are much larger in proportion to the body (the latter especially) than in 

 any others. The skull is narrow ; the width, considerably less than half the length. The 

 incisors of a skull of L. aquaticus, an animal apparently not heavier than L. americanus, exceed 

 those of any I have ever seen, excepting one very large Lepus timidus. Another striking pecu 

 liarity is the entire fusion of the postero-internal margin of the post-orbital process of the 

 parietal bone with the cranium so completely that the suture is not even visible. There is, 

 therefore, neither notch, nor foramen, though the latter is indicated by a fossa. In one or two 

 cases there is a small perforation in this fossa, but very different in character from the foramina 

 of the other groups. 



In addition to these prominent points of difference, there are some others : such as the shape 

 of the lower jaw, the shape and size of the superior outline of the occipital bone, &c. I have, 

 however, said enough to indicate the general fact of a relationship between peculiarities of skull 

 and external form. The principal features, however, as described, will be brought out more 

 prominently by the following synopsis and table of measurements of sixteen species : 



A. Skull very broad and high ; much curved ; the muzzle short ; higher than wide, anterior to the molars. 



Post-orbital processes very large, widely divergent ; bornding a wide notch L. timidus. 



borealis. 

 glacialis. 

 americanus. 

 icashingtonii. 



campestris. 

 B Skull narrower ; less curved above ; muzzle wider than high. Post-orbital processes very large, more 



parallel ; the posterior extremities nearly or quite in contact with the skull, but without complete 



fusion . the foramen enclosed, long and wide /. calif ornicus. 



callotis. 



