095 



Fig. 136. 

 Microtus oreadensis. 



widely .spreading zygomatic arche.s, the .skidl differs little from 



that of typical Microtus arvalis. Central portion of zygoma 



noticeably widened, a character ap- 

 preciable in individuals that are still 



evidently sub-adult and giving the 



bone a decidedly different appearance 



from the slender arch of arvalis. 



Interparietal with longitudinal dia- 

 meter greater and lateral diameter 



less than in arvalis, the resulting 



form more sub-quadrate than in the 



smaller animal. Median ridge of 



posterior termination of palate almost 



invariably with evident groove along 



suture. Tooth-rows relatively 



smaller and less widely separated 



than in 3[. arvalis, the distance be- 

 tween them the same as in the smaller 



species. Auditory bullte rather small 



relatively to size of skull, their actual 



measurements not much greater than 



in M. arralis. 



Teeth. — In structure the teeth do not differ appreciably from 



those of Microtus arvalis, though the enamel pattern in general is 

 somewhat more compact, owing to the rela- 

 tively less width of re-entrant angles and 

 greater area of ti'iangles. Elements of enamel 

 pattern exactly as in M. arvalis ; m'^ without 

 trace of postero-internal loop ; m^ with an- 

 terior loop deeply cut on outer side by a 

 re-entrant angle approximately as well de- 

 veloped as that of inner side, a character 

 that shows no appreciable variation in a series 

 of fifty-three skulls. 



Measurements. — External measurements of 

 type (old male), from skin : tail, 32± ; hind 

 foot, 18. Average and extremes of ten adult 



Enamel pattern, x 5. ™ales from Pomona Island : head and body, 

 118 (113-125); tail, 38-5 (35-44); hind foot, 



18-7 (18—20); ear, 12 (11-13). Average and extremes of .six 



females from Pomona Island: head and body, 117 (114-125); 



tail, 38-4 (34 -5-42 -5); hind foot, 18-6 (18-5-19) ; ear, 11-5 



(11-12). For cranial measurements see Table, p. 699. 



Fig. 137. 

 Microtus oreadensis. 



SjJccimcns examined. — Eighty, from the following islands of the 

 South Orkney group : Rousay, 7 (B.M. and Edinburgh) ; Pomona, 64 

 {B.M., U.S.N.M. and Edinburgh); Shapinsay, 6 (Edinburgh); South 

 Ronaldshay, 3. 



BemarJis. — From the other European voles with similar 



