58 



MICROTIN^ 



bullae have only a weak development of spongy bone within. 

 The dentition is essentially as in Pitymys, m^ difiering from that 

 of Microtus in having only three closed triangles in front of the 

 posterior loop. 



Tyrrhenicola, a fossil genus known from the Pleistocene 

 deposits of some of the Mediterranean islands, is very closely 

 related to Neodon. Only one well-marked species is known, a 

 large vole with a skull in which the facial portion is unusually 

 long and narrow, and the braincase is lofty and subcylindrical. 

 The temporal muscles were evidently powerfully developed, the 

 temporal ridges fusing in the much constricted interorbital region 

 to form a median linear crest; the post-orbital processes of the 

 squamosals are moderately developed. The pterygoid fossae are 

 very deep, indicating powerful pterygoid muscles. The palate 

 is highly specialized, resembling that of " Stenocranius " in the 



Fig. 30. — Cheek-teeth of Pedomys haydenii Baird. 

 Crown views : a. right upper, 6. left lower molars. 



genus Microtus. The auditory bullae are small. The incisors 

 are rather slender, but are normal in other respects. The cheek- 

 teeth are persistently growing, and in other respects are essentially 

 like those of Pitymys and Neodon. 



Pedomys is a North American genus apparently closely 

 related to Neodon. It is slightly more highly specialized than the 

 latter externally, having long, coarse, not specially modified 

 fur, small ears, concealed in the fur, but provided with a large 

 antitragus, broad hands and feet, with the hind-claws slightly 

 longer than those of the hand, with five plantar tubercles, and 

 with hairy soles; the tail is short; the mammary formula is 

 reduced to 1 — 2 =^ 6. The skull and teeth are essentially as 

 in Neodon, but the mastoid portions of the rather small auditory 

 buUaj are considerably inflated. 



Orthriomys and Herpetomys are two very closely related 

 genera, each represented by a single species of restricted dis- 

 tribution, the one stranded upon Mount Zempoaltepee, Mexico, 



