695 



Fig. 136. 

 Microttts orcadensix. Nat. size 



widely spreading zygomatic arches, the skull difters 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 difterent 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 bulla; 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 31icrotiis 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-^ 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 

 males 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. 



Microtus orcadensh 

 Enamel pattern. X 



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, G (Edinburgh); South 

 Ronaldshay, 3. 



BemarJcs. — From the other European voles with similar 



