MICROTUS 



695 



widely spreading zygomatic arches, the skull 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 M. arvalis, the distance be- 

 tween them the same as in the smaller 



species. Auditory bullae rather small 



relatively to size of skull, their actual 



measurements not much greater than 



in M. arvalis. 



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 triangles. Elements of enamel 

 pattern exactly as in M. arvalis; m? without 

 trace of postero-internal loop ; m x 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 



x 5 males from Pomona Island : head and body, 



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



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



Via. 136. 

 Microtus orcadeth&it,. Nat. size 



FIG. 137. 



Microtus orcadensis. 

 Enamel pattern. 



18-7 



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). Eor cranial measurements see Table, p. 699. 



Specimens 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 

 Bonaldshay, 3. 



Remarks. — From the other European voles with similar 



