RODENTIA ARVICOLINAE ARVICOLA. 515 



tt 



positively distinct in the collections before me, without any special synoptical arrangement, as 

 as in other genera. Much more material and labor will be required before this can be done 

 properly. 



In giving the subdivisions of Arvicola, I shall precede them with a brief diagnosis of Hypu- 

 daeus, referring for particulars to the preceding pages. It is to be understood that, though no 

 special prominence is given in the descriptions, yet that Hypudaeus is much more distinct from 

 sections A, B, C, and D, than these are from each other, and consequently more likely to be 

 erected into generic rank. 



HYPUDAEUS, 111. 1 



Ears large, broad, and prominent Margins not inflexcd. Antitragus moderate, though capable of closing the meatus. 

 Fore feet about half the length of the hinder. Soles with six tubercles, the posterior rather small. Tail about two and a 

 half times the length of the hind foot ; considerably longer than the head. Female with four pairs of teats, two pectoral 

 and two inguinal. 



Further details concerning this section will be found in the preceding pages, where its charac 

 ters are compared with those of the restricted genus Arvicola. But one species, H. gapperi, is 

 thus far known in North America. It constitutes an excellent connecting link between the 

 arvicoline Hesperomy* and Sigmodon and the Arvicolae proper. 



ARVICOLA. A. HEMIOTOMYS, Selys. 2 



Ears large and broad, with a highly developed antitragus. Feet large. Anterior rather less than half the length of 

 posterior. Claws moderate, the anterior not longer. Soles with six tubercles ; the posterior very large. Females with 

 four pairs of teats, two pectoral and two inguinal. Second upper molar with two internal triangles, third with two external. 

 Anterior lower molar with three internal triangles, and two or three external. 



Embracing the largest American species. Hind feet very long. Toes long, inserted obliquely 

 on the metatarsus, as in the muskrat, for swimming. Fore feet a little more than half as 

 long as the hinder ; their claws not longer than the hinder ; posterior third of sole hairy. 

 Ears large, though more or less concealed by the fur, coated with long hairs. Antitragus a 

 large, almost semi-circular, valve, capable of closing the meatus. Tail considerably longer than 

 the head, usually about one-third the head and body. Plumbeous portion of the hairs uniform 

 throughout. Tail not darker at tip. 



Skull rather narrow. Interparietal bone truncate laterally. Nasal branch of intermaxillary 

 very little longer than the nasal bone, and barely reaching to the interorbital region. Second 

 upper molar with five closed triangles ; the two last sometimes sub-confluent. The third upper 

 molar with one anterior, two external, and one internal closed triangles, and a posterior C-shaped 

 or sub-crescentic loop, the convexity external, sometimes with an additional internal spur. 

 Anterior lower molar with one posterior triangle, three internal, and two or three external, 

 with also an anterior trefoil loop. When there are three external triangles, the outer lobe of 

 the loop is changed to a triangle. Middle lower molar with five triangles. 



The shape of the crescent loop of the posterior upper molar may vary in the same species ; 

 sometimes with two lobes only, sometimes with a third inner lobe, making three salient inner 

 angles. Sometimes the posterior lobe is not bent inwards, but is simply directed backwards, 



1 Myodes of Selys Longchamps, Etudes, 87. 



* Hemiotomys, Selys Longchamps, Etudes de Micromammalogie, 1839, 85. Constructed to accommodate the European 

 Arvicola amphiUm, (crreslris, &c. The skull of these, however, is more like that of section C. 



