RODEJVTIA ARVICOLINAE ARVICOLA. 6lB 



Although I have not had the opportunity of examining all the American Arvicolae, they will, 

 probably, all be found in one or other of several groups which will hereafter be characterized. 

 Externally, the differences are in the size and shape of the feet and ears, the length of the 

 tail, the condition of the fur, &c. The variations of the skull have already been referred to. 



Hypudaeus. The description of Arvicola has necessarily involved that of many points 

 common to it and Hypudaeus, and it will, therefore, here be only necessary to advert to the 

 characteristic features. The most striking of these is seen in the molars, which, in the adult, 

 have two fangs or roots, each separated by a slit sometimes nearly half the length of the tooth. 

 Some other points, to which attention may be called, are as follows : The nasal bones and nasal 

 branches of the intermaxillary are of equal length, and scarcely extend beyond the anterior 

 insertion of the palatine arch, never to the interorbital region. The interparietal bone is acute 

 angled laterally, not truncate. There is a slight concavity in the top of the head anterior to 

 the parietal bones. The posterior end of the bony palate is a projecting shelf, with the wings 

 of the sphenoid and palatine bones dipping and disappearing under it by sinking to the level 

 of the base of the head. There is, therefore, no mere fossa on each side of the anterior molars, 

 but the whole intermolar space is broadly excavated and open. The incisive foramina are long 

 and wide. The coronoid process of the lower jaw is long but low, and does not reach to the 

 level of the condyle. 



The anterior upper and posterior lower molars are much as in Arvicola. The middle 

 upper molar has one internal and two external triangles. The posterior molar is shaped much 

 as in the Arvicola riparius section, viz : one anterior triangle, two external and one internal, 

 with a posterior crescent, the two loops of which form the two last internal salient angles. 

 The corners of the angles are all rounded off, however. The middle lower molar, like the 

 posterior, is composed of three transverse triangles, although it is considerably wider. The third 

 molar has a posterior transverse triangle, and the lateral salient angles are more or less opposite 

 each other, so that their dentine is confluent with transverse triangles ; there are usually two of 

 these lateral open triangles on each side, and a trefoil loop anterior to all. 



The incisors are narrower than usual ; the entire skull narrow and depressed. The line of 

 molars is very short and narrow, not more than one-fifth the length of the skull. 



Thus far, I have only found one, perhaps two, species of Hypudaeus in America. There are, 

 however, several in Europe, in addition to the typical H. glareola, upon which the sub-genus 

 was founded. Among them are H. rubida and H. rutila of northern Europe, so similar to 

 A. gapperi. Arvicola arvalis of southern Europe, and Arvicola agrestis, of northern Europe, 

 supposed to be identical with A. arvalis of authors, fall legitimately in the other genus. 



To sum up, therefore, in a few words, the characters of Arvicola and Hypudaeus, we have the 

 following diagnoses : 



ARVICOLA. Molars rootless. Bony palate with a fossa on the inner side of each posterior molar, and between these fossae 

 an opening to the basi-sphenoid one-third the intermolar space. Coronoid process as high as the condyle of the lower jaw. 

 Ears more or less concealed, coated with rather loose hairs. 



HYPUDAEUS. Molars with two roots each. Bony palate behind a projecting shelf ; no fossae, but the entire space between 

 the posterior molars excavated and open to the basi-sphenoid. Coronoid process not as high as the condyloid. Ears 

 very distinct, covered with short close hairs. 



Although, by defining Hypudaeus as distinct from Arvicola, the labor of ascertaining the 



species is somewhat lessened, still enough are left in the latter to render it very desirable to 



construct further subdivisions. I have, after a somewhat protracted examination, succeeded in 



finding characters by which the Arvicola may be thrown into five or six smaller groups, each 



65 L 



