ZAPODIDJ3— ZAPUS HUDSONIUS— CRANIAL CHARACTERS. 469 



supplementary foramen or nick just beneath ; the extension of the malar 

 bone up the slender styloid zygomatic portion of the maxillary till it sutures 

 with the lachrymal, and the slenderness and depression of the rest of the 

 zygomatic arch ; the shortness and transverse position of the bullae auditorise; 

 the position of the maxillo-palatine suture; expansion of the posterior nares, 

 &c. The skull, as a whole, is shorter for its width, though the zygomata are 

 more nearly parallel ; it is also deeper for its other dimensions, with a greater 

 degree of convexity, both lengthwise and crosswise, of the superior contour. 

 Nevertheless, its general superficial resemblance to that of Mus proper is evi- 

 dent. Compared with that of Mus musculus, which is of about the same size, 

 we see in each species the same general shape and delicate papery condition, 

 without strong angularity, as well as many close coincidences in detail, indi- 

 cating that the Murine affinities of the animal are with typical Mus, Hespero- 

 mys, &c, and not with the Arvicoline group of Muridcn, in which the skull is 

 notably heavier, more massive, and more angular. 



As to the general shape of the skull, there is little to be added to the 

 foregoing, except such points as, being equally applicable to the familiar Mus 

 musculus, need not be recapitulated ; we may therefore at once proceed to 

 details, in giving which I frequently compare the skull with that of Mus to 

 indicate its departures from ordinary Murine features. 



The anteorbital foramen, which transmits the masseter in this instance, 

 and which constitutes a prime peculiarity of the skull, is of great size and 

 obliquely oval in shape. Instead of being circumscribed by a plate of bone, 

 as in Muridce, it is defined externally by a very slender styloid process of the 

 maxillary, which is strengthened by the upward extension of the malar, 

 applied as a splint along its whole length. Below this main foramen there is 

 another much smaller one, which transmits the nerve. This is sometimes a 

 complete foramen, separate from the other; sometimes only a deep notch in 

 the lower bolder of the main opening; and this difference may be observed 

 on the two sides of the same skull. I am ready to believe that this lesser 

 opening, giving passage to the superior maxillary nerve, is the true "anteorb- 

 ital" foramen itself; for it seems to correspond to the lower part of the large 

 slit which, in Muridce, is walled in by the maxillary lamina, and it is formed 

 by a little plate of bone, which rises as a ridge from the alveolar portion of 

 the jaw, and bends over to abut against the main wall of the maxillary. In 

 cases in which this plate lails to reach the main wall of the maxillary, so that 



