556 U. 8. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



There are two re-entrant and three salient angles on each side. The middle molar has four 

 narrow elongated triangles the first and second entirely transverse ; the third internal ; the 

 fourth external. The first and second triangles are in contact on the inner edge of the tooth. 

 There are three external and two internal salient angles, and two re-entrant external, one 

 internal. The posterior molar has four triangles, the two anterior entirely transverse, and 

 united on the inner edge of the inolar, as in the middle molar ; the two posterior triangles 

 likewise transverse, but united on the exterior of the tube ; the stem connecting the second and 

 third a little exterior to the middle. There are three salient angles, and two re-entrant on 

 each side. 



The lower anterior molar has one posterior, one external, and two internal triangles, and 

 an anterior small quadrangular loop. The middle molar is very similar, the anterior 

 quadrangular loop, however, replaced by an internal triangle, making two lateral on each side, 

 or five in all. The posterior has four triangles, the two posterior entirely transverse, and con 

 nected on the exterior of the tooth ; the outlines of each distinct. The other triangles are 

 lateral on either side. 



To sum up the whole statement in reference to the molars, the anterior in both jaws have 

 five prisms, each with closed triangular or sub-triangular sections ; the posterior with four ; 

 the middle upper has four, the middle lower five. The posterior molars are as wide as the 

 anterior. 



Myodes scMsticolor differs a little from . lemmus n a less hairy sole, smaller fore claws, a 

 r ather narrower head, pentagonal interparietal, and a few other minor features of skull and teeth, 

 to which I need not here enumerate. Both have the palate ending as in Hypudaeus, not Arvicola. 



By the preceding description it will be seen that there are many points of difference between 

 the skulls of Myodes and Arvicola, as in the former we have a much greater width of head, 

 more contracted interorbital space, longer interparietal, deeper zygomatic arch. In the teeth 

 the differences are very striking, especially in the depth of most of the re-entrant angles, which 

 so often extend to the opposite side of the tooth. The posterior lower molar is composed of 

 four sub-triangular prisms, not three, the anterior lower of five, while in Arvicola the number 

 is always much greater. 



I have given a minutely detailed description of the external anatomy of a European species 

 of Myodes, (M. lemmus or Lemmus norvegicus of authors,) for the sake of illustrating a very 

 remarkable specimen received from Mr. William Cooper, of Hoboken, and having all the external 

 appearance of an Arvicola. In fact, truly a Myodes in its teeth, it constitutes a perfect link 

 connecting the two genera. I cannot now say whether any of the Lemmings of arctic America 

 are like it or not, but judge from the descriptions that they are all true Myodes. 



The specimen in question, (No. 1367,) is in very imperfect condition, the tail, skin of the 

 head, and all the feet, except one of the anterior, being deficient. A smaller skin, (1368,) 

 lacking both the skin of the head and the skull, accompanied it, and probably belongs to the 

 same species. No locality was assigned, but the animal is undoubtedly North American, prob 

 ably from the New England States, or New York ; possibly from Iowa or Minnesota. 



The size is a little less than that of A. riparia. The fur is remarkably soft and full, over 

 half an inch long, and has longer black hairs interspersed. The color is a light yellowish 

 brown above, relieved by black, precisely as in Arvicola; the sides are rather paler ; the under- 

 parts are hoary white. 



The single remaining fore foot has lost the palm and the thumb claw, Ihe rest of the hand is, 



