M. A. Milne-Edwards on the Group of the Mole-Bats. 159 



gree in which the tooth is worn. It is only necessary to 

 follow these changes to become convinced that the supposed 

 Zocor figured by F. Cuvier and by M. P. Gervais is only a 

 young Zemmi, and that the true Zemmi of these authors is an 

 adult or even aged individual of the same species. 



The molars of Siphneus belong to quite a different type. 

 There are three pairs of them in each jaw ; but they never ex- 

 hibit roots, whatever be the age of the animal ; consequently 

 their growth is unlimited, and their form does not become 

 modified, whatever be the degree of wearing of the tooth. 

 They are formed by more or less triangular prisms alternating 

 in an irregular manner, and resemble those of the Arvicolce in 

 all the essential features of their construction. This approxi- 

 mation, moreover, agrees very well with the other characters 

 of the Zocors and the Voles. 



These considerations lead me to refer the genus Siphneus 

 to the little division of the Arvicolino?, of which it may be re- 

 garded as an essentially fossorial derivative type, modified in 

 its external form in consequence of the conditions in the midst 

 of which it has to live. The genus Elobius (Mus talpinus, 

 Pall.) must take its place in the same family. On the con- 

 trary the Zemmis (genus Spalax) belong to the group of Mole- 

 Rats properly so called, of which Bathyergus and Georychus 

 are the principal representatives. 



Hitherto only a single species of Siphneus has been known ; 

 this came from Siberia. The Museum of Natural History has 

 just received two others, completely unknown to zoologists, — 

 one of them {Siphneus Fontanierii) discovered in the neigh- 

 bourhood of Pekin ; the other (S. Armandii) has hitherto 

 been found only in Mongolia. These Rodents are very diffi- 

 cult to distinguish by their external characters, but they may 

 be determined at once and with certainty by the examination 

 of their dental system. I cannot enter here into an examina- 

 tion of the anatomical details by means of which this end may 

 be attained, and I shall merely refer the reader to the more 

 complete memoir that I have prepared upon this subject. 



I will add that in the caverns of Siberia bones of Siphneus 

 myospalax are found, as I have been able to ascertain by the 

 study of the specimens derived from excavations made on the 

 banks of the Inia and Tcharysh by MM. Meynier and L. von 

 Eichthal, which have been kindly communicated to me by the 

 latter. In Mongolia the Abbe Armand David collected several 

 skulls belonging to Siphneus Fontanierii and S. Armandii in 

 alluvial deposits probably of quaternary age. At this ancient 

 period, therefore, the geographical distribution of the species of 

 this genus was the same as in the present day. 



