294 Prof. A. Nehring on Myodes Icimnus crassidens. 



coronoideus of tlie under jaw and the greater breadth of the 

 cheek-teeth. Some slight differences also appear to exist in 

 the bones of the extremities. 



As is well known, ]\Iyodes lemmus^ L., M. schisU'color , 

 Lilljeb., and M. obensis, Brts., form a group of species of 

 lemmings whicli almost exactly agree one with another in the 

 shape of the check-teeth. They can be placed together under 

 the generic name of Lemmus, Link. Especially characteristic 

 of them is the shape of the third lower molar (???. 3 inf.) : the 

 latter shows four distinctly separate prisms on the masticatory 

 surfaces, while in 3f. torquatiis there are five distinct prisms, 

 and the species of Arvicola and Microtus have only three* 

 left. The remaining cheek-teeth also in the species of 

 lemmings have such a characteristic shape that an expert 

 can instantly determine them as belonging to Lemmus. This 

 refers particularly to the last upper molar {m. 8 sup.), as well 

 as to the relationship of the outer and inner enamel prisms 

 one to another in m. 1 and m. 2 sup. The masticatory surfaces 

 of the lemming's teeth seen in figure 1 may be compared 

 with those of Arvicola given in figure 2. I have placed 

 the two side by side for purposes of comparison. 



A distinguishing feature of the lemmings may be seen in 

 the length and position of the socket of the lower incisor. 

 This socket runs along the inner wall of the under jaw, 

 near the sockets of m. 1 and m. 2, and ends before reaching 

 that of m, 3. In the genus Arvicola it extends from the 

 inner side of the jaw, slopes under m. 2 towards the outer 

 wall of the same, and rises a considerable distance into the 

 articulation process {condylar process) above, the end of the 

 socket being marked on the outside by a sort of knobf. Myodes 

 torquatus shows almost exactly the same form in the socket 

 of the lower incisor as in tlie species of Lemmus, but this 

 runs back past the first, still nearer to m. 3, and is thus 

 somewhat longer. 



Very characteristic of Myodes lemmus, J/, ohensis, and 

 M. schisticolor is the relatively broad flat form of the skull, 

 with which a well-marked j)rojection of the jugal is combined 

 (see fig. 3). It is only necessary to place the skull of one of 



* See my detailed paper on "Fossil Lenuiiiugs and Arvicolcf!" in 

 Giebel's ' Zeitschr. f. d. Natinwis8.' 1875, vol. xlv. pp. 1-28, pi. 1. Here 

 llie differences occurring in the construction of the cheek-teeth of 

 M. lemmus, M. tonjuotiis, and the largest of the species of Arvicola are 

 described. Compare also my paper on the " Smaller Vertebrates of 

 Schweizersbild, near Schaffhausen," reprinted from the ' Denkscbr. d. 

 Schweizersbild. naturf. Ges.' vol. xxxviii. p. 23 et seq. 



t Compare my account, op. cxt. p. 23. 



