46 MICROTIN^ 



the teeth in individuals so young that they have not yet fully 

 acquired the adult pelage. In pattern the teeth are not very 

 different from those of Evotomys ; but mj, which possesses five 

 alternating triangles between the posterior and anterior loops, 

 shows in young stages of wear some vestigial and ephemeral 

 complications of the anterior loop ; whereas m^ is reduced and 

 has only three or four outer and three inner salient angles. 

 Cement is present in the re-entrant folds of the teeth in the 

 recent species, but is not developed in those of the Pliocene forms. 



The genus is most satisfactorily distinguished from Evotomys 

 and placed upon a higher plane by its skull. The hinder portion 

 of the palate shows a very broad and ill-defined median slop- 

 ing septum, small, shallow and indefinite postero-lateral pits, 

 and more or less incompletely developed lateral bridges. In 

 Dolomys, therefore, we see the beginnings of that palatal struc- 

 ture, which is better developed in Arvicola, and becomes 

 perfected in the most specialized species of Microtus. The 

 temporal ridges, in the recent species at all events, fuse anteriorly 

 to form a median interorbital crest in adults and posteriorly 

 they are fairly closely approximated. The alveolar capsules of 

 the cheek-teeth do not rise up in the floor of the orbit or in the 

 sphenorbital fissure. The auditory bullae lack internal spongy 

 tissue, but they are large and considerably inflated; the 

 stapedial artery is enclosed in a bony tube as far as the stapes. 

 The mandible is normal; m^ is not noticeably displaced by 

 the shaft of the incisor. The upper incisors are strongly curved 

 and sometimes show a slight trace of an anterior groove. 



Apistomys, described from the Upper Pliocene of Hungary 

 and known only from fragmentary remains, is scarcely to be 

 distinguished as a genus from Dolomys. In it m^ has a general 

 " arvaloid " appearance, whereas in Dolomys this tooth is more 

 like that of some forms of Evotomys or Microtus nivalis in general 

 form. 



MiMOMYS is a very interesting and important genus, repre- 

 sented by numerous species in the Upper Pliocene of Europe and 

 by one species in the early Pleistocene of Britain. It appears to 

 have descended from some primitive species of Dolomys, i.e., 

 from a form more primitive in skull structure than is the only 

 living representative of the latter genus. So far as is known, the 

 temporal ridges remained rather widely separated in the inter- 

 orbital region in Mimomys, instead of fusing into a weak linear 

 crest as in recent Dolomys. But in other respects Mimomys has 

 gone much further than Dolomys, and in the later Pliocene and 

 earlier Pleistocene deposits of Britain species of Mimomys shade 

 off imperceptibly into forms which cannot be distinguished with 

 available materials from the genus Arvicola. The palate is 

 essentially as in Arvicola, its postero-median sloping septum 

 being short, broad, but rather well defined. 



As in Dolomys the cheek-teeth of Mimomys develop roots in 



