662 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



In Zapus the skull has a decided murine 

 aspect, long and slender, with an unex- 

 panded brain-case, no mastoid bullje, and 

 the zygoma sloping downward and back- 

 ward from the maxilla. 



In Diptis the skull has no murine aspect 

 whatever; it is broad and heavy, much 

 expanded behind, Avith the mastoid bones 

 inflated as large as the true bullae and the 

 outer border of the antorbital foramen 

 standing out in wing-like projections and 

 sloping downward and slightly forward. 



The palatal and pterygoid reg-ion.s are quite different in the two 

 genera. 



In Zapus the palate bones are much 

 shortened posteriorly, the free edge con- 

 cave and ending on a line with the last 

 molar teeth. It shows exactly the same 

 condition as is found in Mas. 



The pterygoids have the usual forms 

 and proportions seen in Mus. 



In Zapus the external pterygoiil plate 

 assumes a more horizontal position and 

 longitudinal direction, as in Mus. 



Between the external plate (a process 

 of the alisphenoid) and the internal 

 pterygoid plate (the true pterygoid bone) 

 is a shallow fossa, entirely destitute of a 

 floor and of the same form as is found in 

 Mus. 



In Dipus the palate is much more 

 elongated and produced jjosteriorly to a 

 considerable distance behind the last 

 molars and ends in a l)lunted projecting 

 spine. 



The greater posterior length of the bony 

 palate makes the pterygoids correspond- 

 ingly shorter. 



In DIpus the external pterygoid plate is 

 more vertical and more transversely 

 placed. 



Between the external and internal 

 pterygoid plates is a deep and conspicuous 

 fossa, running forward and being floored 

 by the posterior lateral portions of the 

 bony palate and having for its roc'f the 

 alisphenoid. It is a fossa on the order of 

 that seen in Microti! s. 



The tympanic bones are of similar form and position in each genus, 

 triangular in outline and placed more transversely than longitudinally, 

 as against the position in the Mnrklp. They are each inflated to form 

 bull», which are — 



I 



smaller in Zajms, not approaching the 

 median line, with the apices free from 

 the basisphenoid. The inner edges abut 

 closely against the basioccipital, so that 

 no vacuities are formed. 



The mastoid portion of the periotic is 

 not abnormally enlarged in Zapus, does 

 not overcrowd any of the other bones, 

 and has a form and i^osition A'ery similar 

 to Mus. 



The squamosal in Zapus is a thin and 

 expanded bone, with its antero-posterior 



larger in Dipus, nearer the median line, 

 and the apex of each is definitely fused 

 to the basisphenoid. Between their inner 

 edges and the l)asioccipital are large 

 vacuities. 



The mastoid portion of the periotic is 

 greatly inflated in Dipus and presents 

 almost as much surface on the posterior 

 part of the skull as the tymj^anic bulla 

 does on the ventral surface. The two 

 portions push Inward to such an extent 

 as to encroach upon the supraoccipital 

 and render that bone correspondingly 

 narrow. They swell out laterally and 

 superiorly, so that a portion is seen above 

 the tympanic and ])etween a posterior 

 process of the squamosal and the jiarietal 

 and supraoccipital. 



Tlie squamosal in Dipus is a compact 

 and much contracted bone, with its dorso- 



