98 MICROTIN^ 



Skull with temporal ridges closely approximated or fusing 

 throughout; squamosals very large, their upper borders 

 tending to be parallel, the lateral wings of the parietals 

 obsolete. 



Highly modified for fossorial life. 



A. Skull of nearly normal general form; interorbital region 



much constricted ; post-orbital crests of squamosals 

 shelf-like though weak; infraorbital canal normal; 

 anterior palatine foramina large; palate abnormal, 

 inner borders of postero-Iateral pits not directly con- 

 nected with palatal shelf, each pit with a large foramen. 

 Auditory bullae moderately large, without spongy 

 tissue, but their walls strengthened by incomplete 

 perpendicular septa ; external meatus tubular ; stapedial 

 artery naked. 



Mandible normal, \vith well-developed angular and 

 obUquely inchned coronoid processes; alveolar pro- 

 tuberance of incisor well below the level of the condyle. 



Upper incisors strongly curved, ending nearly flush with 

 nasals in front and terminating in maxilla just in front 

 of m^ behind, broad, each with a weak anterior groove ; 

 enamel of incisors yellow, m^ bilobular, vrith three 

 very small outer and two large inner salient angles, its 

 first infold very shallow leaving the first outer triangle 

 broadly confluent with the anterior loop ; mj with three 

 outer and four inner salient angles, and three alternating 

 substantially closed triangles; m^ bilobular with two 

 outer and three inner saUent angles, the first inner infold 

 shallow leaving the posterior loop and first triangle 

 broadly confluent, the third outer saUent angle obsolete. 



Fur long and soft; eyes small; ears moderately 

 large, not concealed; hands with enormous claws, 

 those of the three central digits especially lengthened, 

 slender, and recurved; feet with large claws but these 

 only about half as long as those of fingers ; palms and 

 soles naked between the pads ; plantar tubercles 5 ; tail 

 short and thick, densely haired. Mammae 2 — 2 = 8. 



Prometheomys. 



B. Skull of extreme " fossorial " form, with straightened and 



far-protruding incisors, shallow and slender rostrum 

 and shghtly inclined occiput ; interorbital region broad, 

 not sharply constricted off from braincase ; post-orbital 

 crests of squamosals weak; infraorbital canal wider 

 above than usual, but lacking the lower fissure-Hke 

 portion seen in Prometheomys and normal voles; 

 anterior palatine foramina very small ; palate essen- 

 tially as in Microtus. Auditory bullae small, their 

 walls strengthened by numerous stout perpendicular 

 trabeculse; external meatus very smaU; stapedial 

 artery enclosed in a bony tube. 



Mandible with nearly vertical coronoid process and 

 much reduced angular process; alveolar protuberance 

 of incisor rising to the level of the condyle. 



Incisors with white enamel ; upper incisors unusually 

 long, slender, and straight (" proodont "), their tips 

 protruding far in front of the nasals, their alveolar 



