NO. 1228. 



OSTEOLOGY OF THE JUMPING MICE— LYON. 



668 



cliameter greater than its dorso- ventral, 

 and is of the same type as is luund intlie 

 Muridtv. 



The zygomatic process of the s<iuauio- 

 sal niucli ex^ianded at its origin vtu-cvx 

 decidedly downward. Tliis is in accord- 

 ance witli its higher origin from tlie bone. 



ventral diameter much greater than its 

 antero-}K)steri(>r. Its shape and j)Osition 

 are diffic-ult to describe, and are best seen 

 in the llgure. 



The zygomatic process of the sipianio- 

 sal comes ont almost horizontally, dojiiiuj 

 a little downward. This is in accord- 

 ance with its lower oriiiin from the bone. 



The zvg-onmtic reo-ion shows seveml dirieroiices. That of ]>ip(tx is 

 apparently an extreme typo, between which and Mux, Zapus seems to 

 be somewhat intermediate. In 2£us and other nnirines, both roots of 

 the zyt,^omati(' proeess of the maxilla (saying that there is an upper 

 root above the antorliital foramen and a lower root below it) arise one 

 directly above the other. This condition holds — ■ 



in Zapus, where the lower root arises just but in Dipus the upper root comes off at 

 in front of the premolar and the upper a consitlerable distance posterior to a 

 root about on a line directly above. This point directly above the lower root. This 



condition causes the anterior part of the 

 zygomatic arch to slope downward and 

 forward. 



DIjitis has a l)iradiate malar, the verti- 

 cal part of which is much expanded lat- 

 erally and tits into a right angle in the 

 maxilla. The horizontal part is slender 

 and runs liackward to meet the squa- 

 mosal. 



Theantorbital foramen in Dipus is more 



condition causes the anterior part of the 

 zygomatic arch to slope from above 

 tlownward and backward. 



Z(tpt(s has an almost triangular malar, 

 which fits into the obtuse angle in the 

 zygomatic process of the maxilla. The 

 lower posterior angle of the malar is 

 attenuated into a slender process going 

 backward to the squamosal. 



The antorbital foramen in Zapus is 

 more nearly elliptical, the major axis of nearly ovoid, and the long axis inclines 

 the ellipse inclining outward from above slightly inward from above downward, 

 downward. 



At the lower inner corner of this foramen i.s a separate canal for the 

 transmission of the second division of the tifth nerve, formed ])y a 

 thin plate of bone arising from the lower root of the zygomatic process 

 and abutting against the outer sitrface of the maxilla. 



In Dipus this plate is completely anchy- 

 losed and the line of fusion obliterated. 



In Zapus the line of contact of this thin 

 plate with the maxilla is always evident, 

 and very often the plate fails to meet the 

 side of the maxilla. 



The wall of the orbito-temporal fossa 

 in Zapus shows a condition such as is 

 found in Mus and the Muridir generally, 

 with all the bones ossified and in close 

 approximation to one another. 



In Dipus the wall of this fossa shows 

 (piite a deficiency in ossification. The 

 optic foramina are unusually large. Just 

 behind the orbito-sphenoid is a large 

 crescent-shaped vacuity bounded in front 

 liy the orbito-sphenoid and the orbital 

 plate of the frontal; above and behind, 

 by the squamosal; and below, by the ali- 

 sphenoid. 



The incisor teeth in both genera are short and curved backward 

 after the manner of the Marldce. Each tooth is traversed by a groove 



