278 PROCEEDINGS OF THE ACADEMY OF [1895. 



anteriorly that the ascending process of the jugal appears to rise out 

 of the middle of the zygomatic arch. The zygomatic process of the 

 maxillary is thus merely a slender frame-work enclosing the infra- 

 orbital foramen and strengthened on the outer side by the ascending 

 process of the jugal. This is quite different from the perforated 

 plate of the true mice and still more from the imperforate plate of 

 the Heteromyidce and Geomyidce. The lachrymal is a very small 

 bone, which is placed at the antero- superior angle of the orbit and 

 articulates with the frontal and the jugal. 



The premaxillaries are large and quite heavy bones, very narrow, 

 but of considerable vertical extent; their palatal surface is reduced 

 to two narrow rods, one on each side of the very large incisive 

 foramina. The maxillo-premaxillary suture is just in advance of the 

 zygomatic process, as is also the case in Dipus and Zapus, and thus 

 the sides of the rostrum are formed almost entirely by the premaxil- 

 lse. The incisive foramina are very large, though the apparent size 

 is increased by the loss of the premaxillary spines; they deeply 

 emarginate the palatal processes of the maxillaries and extend back 

 as far as ml. The John Day genus Paciculus and several of the 

 recent Dipodidce, such as Zapus, Dipus, Alactaga, etc., have a similar 

 conformation of the incisive foramina, but in the latter they do not 

 cut so deeply into the maxillaries. This appears to be due to the 

 growth of the part of the maxillaries in front of the premolar which 

 accompanies the forward shifting of the zygomatic process, in Pedetes, 

 in which this shifting has attained its maximum, the foramina have 

 become quite small. 



The maxillaries form singularly little of the facial region. The 

 alveolar portion is low, but quite dense and heavy and forms 

 posteriorly a broad, shelf-like floor of the orbit, which is much larger 

 and heavier than in the recent genera, The bony palate is short 

 from before backward, quite broad and gently arched from side to 

 side. The limits of the palatine bones cannot be made out in the 

 specimen. The posterior nares have much the same shape and 

 position as in Zapus, being broad and extending forward to the 

 middle of mo. The pterygoids abut against the tympanies and 

 there are quite large pterygoid fossa 1 . 



The cranial foramina are not satisfactorily displayed in the speci- 

 men. A large sphenoid tissure is visible in the orbit, but the 

 alisphenoid canal, if originally present, has been destroyed. 



