MURID^— SIGMODOJJTBS— NEOTOMA. 







appearances arc obscure or lost. Upper molars mostly 3-rooted ; under with 



only two roots apiece. 



- In amplification of the characters of this genus, we continue: 



Ncotoma shows us the largest murine skull of North America, and many 

 strong generic characters. The zygomatic width is relatively greater than in 

 other forms, amounting to half the length of the skull, mainly in consequence 

 of the outward obliquity of the zygomata, which diverge from each otiicr as 

 they pass backward, instead of lying nearly parallel. The same length 

 of skull, with parallel zygomata, would much exceed twice the zygomatic 

 width. They also do not dip so low down as in most other genera, not descend- 

 ing to the level of the palate ; and the union of the jugals with the squamosal 

 spurs is abrupt, almost angular. In general contour, there is an observable 

 difference in the two sections of the genus — that with scant-haired tail and 

 the bushy-tailed species ; the latter having the rostral portion of the skull 

 more lengthened and the internrbital constriction greater. In N. Jloridana 

 the interorbital width is about one-seventh of the total length, and rather 

 exceeds the rostral diameter, though this is swollen. The nasals are much 

 widened anteriorly, where they end with crescentic outline; behind, they 

 stop opposite the anterior roots of the zygomata, and always (we believe) in 

 advance of the nasal branches of the intermaxillaries, which reach into the 

 interorbital constriction. In most other genera, there is no special difference 

 between the length of the nasals and the intermaxillary behind, or else the 

 difference is fluctuating. At the extreme antero-superior corner of the 

 orbit, the maxillary, just where it unites with the frontal, throws up a tubercle 

 for muscular attachment stronger than we have noticed in other genera. The 

 thin plate of the superior maxillary that forms the outer wall of the anteor- 

 bital foramen has a gently-rounded anterior borddr, with its convexity looking 

 forward, instead of running into a sharp point as in Sigmodon, where the 

 same edge is deeply concave by reason of this pointed proi >s ; and likewise, 

 when looked down upon from above, the foramen appears as merely an emar- 

 gination instead of a nearly-closed oval. The edge of the orbit is sharp 

 above, but not beaded. The interparietal is moderate and subquadrate, with 

 a spur; the paroccipital processes are strong; the tympanic bulla?, medium; 

 the pterygoids large, hamulate, and fenestrate. The foramen magnum is 

 remarkable for its depressed elliptical shape, being usually much wider than 

 high ; the condyles are far apart at the ends of the major axis. The ))alatc 

 ends behind as a simple shelf with concave border opposite the last molars 



