MURIDAE—SIGMODONTES—HESPEROMYS. AT 
so constricted across the orbits, and having a rather shorter and more swollen 
rostral portion ; the molars are a little larger, with more open indentations 
of enamel along the sides. The skull of Oryzomys enlarges a little upon that 
of Onychomys, and has the sharp edge of the orbits raised into a slight thin 
crest ; there is also some difference in the palate, as described beyond. But 
these differences are all so slight, that, were they unaccompanied by striking 
external characters, they could hardly be considered as of more than specific 
value. In the following description of the skull of Vesperimus, the cranial 
characters of all North American Hesperomys will be essentially reflected. 
The skull is thin and papery, showing nothing of the solidity and massive- 
ness and strong ridges of Arvicoline. The cranial part is broad and depressed ; 
the lengthwise profile of the top is one very gentle curve, both behind and 
before, from the highest point opposite the orbits. The zygomatic width is 
almost precisely half the total length; the height of the cranium is three- 
eighths of the total length; the length of the lower jaw is two-thirds of the 
total length. The zygomata, very slender and strictly styloid, dip deeply 
down to the level of the palate. At first, they stand out at right angles with 
the skull, then sweep abruptly backward till they become parallel, and then 
turn abruptly up to the squamosal. Nearly all the arch is made by the large 
processes of the maxillary aud squamosal; the jugal itself being extremely 
minute. The cranial part of the skull does not noticeably encroach upon the 
orbits, which are rather shallow subcircular cups, with the principal foramina 
crowded into a depressed corner low down and far back, just above the alve- 
olar level—in fact, a little behind (if anything) the last molar. The interor- 
bital constriction is moderate, but always wider than the rather slender and 
tapering rostrum. The nasal bones stick far out in front, with the intermax- 
illaries, surpassing a perpendicular let full upon the faces of the incisors; 
behind, these bones are likewise subequal, and they nearly or quite reach to 
opposite the orbits (they vary a good deal in different species, as well as in 
different specimens of the same species, in this regard). The foramen magnum 
is large, subcircular, or trefoil from emargination superiorly. The parietals 
are small and subquadrate; the interparietal is small and short for its width ; the 
bullae osseze are small, very thin, and very obliquely placed, owing to the 
wedge-shape of the basi-occipital. The incisive foramina are rather open, but 
short, ending in advance of the molars. The palate ends behind almost 
exactly as in Mus, as far as configuration is concerned; but it does not reach 
