MUllIDAE— 8IGMODONTE8— IlKSl'KltOMYa 



47 



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. Tiie skull of Onjzomys enhirges a little upon that 

 of Omjchomys, 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 tlie 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 Arvicolina. 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 lengtii ; the length of tlie 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 nght angles with 

 the skull, then sweep abruptly backward li'l they become parallel, and then 

 turn abruptly up to liie squamosal. Nearly all the arch is made by the large 

 processes of the maxillary and 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. Tiie 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 o£ 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 specimen^ 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 

 bulljB osseae are small, very thin, and very obliquely placed, owing to the 

 wedge-shape of the basi-occipital. The incisive foramina arc rather open, but 

 short, ending in advance of the molars. The palate ends behind almost 

 exactly as in Afus, as far as configuration is concerned ; but it does not reach 



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