M1CR0TU9 



715 



brain-case scarcely rising above occipital level ; paroccipital pro- 

 cesses rather short, their tips projecting only a little beyond 

 level of lower lip of foramen magnum, the ridges extending 

 upward from their bases evident though not very high. Floor 

 of brain-case nearly smooth, the median ridge of basioccipital 

 slightly developed ; width of basioccipital along anterior suture 

 contained about 21 times in median length ; auditory bulke large 

 but somewhat flattened, otherwise without special peculiarities. 

 Interorbital region rather wide (usually wider than rostrum) and 

 short, the upper surface flattened and with low, inconspicuous 

 temporal ridges which may come together very late in life, but 

 which are too low to form an evident median ridge ; inter- 

 lachrymal region always essentially smooth. Zygomata not very 

 widely or abruptly spreading, the contrast between zygomatic 

 breadth and breadth of brain-case much less than in Microtus 

 agresfis ; median expansion evident but not very wide ; anteorbital 

 foramen rather wide and low, its form diagnostic as compared 

 with that of M. agrestis. Rostrum slender, relatively smaller 

 than in M. agrestis and with less contrast in depth between 

 proximal and distal regions, the least depth behind nasals 

 distinctly greater than width in same region ; nasals less narrowed 

 posteriorly than in M. agrestis, their hinder border usually 

 truncate and scarcely extending behind level of anterior border 

 of zygomatic root ; nasal branches of premaxillaries usually 

 extending about to level of middle of zygomatic root ; incisive 

 foramina about as in M. agrestis, but extending backward 

 scarcely to level of m^. Palate narrow, its structure normal, but 

 all the surface sculpture lacking in relief, the sloping portion of 

 median ridge wider than small, shallow lateral pit ; mesopterj-goid 

 space nearly parallel sided, its anterior border rounded, its width 

 in front scarcely equal to distance from margin of fossa to 

 alveolus of iti^ ; plates forming outer border of space nearly 

 vertical ; hamulars straight ; ectopterygoid plate well developed, 

 the ectopterygoid pit large and deep, though slightly more 

 encroached on than is the case in 31. agrestis. Mandible 

 essentially as in M. agrestis but more slender, coronoid process 

 lower, and angular process longer. 



Teeth. — Incisors essentially as in Microtus agrestis but 

 relatively a little less robust ; upper teeth projecting slightly 

 forward, so that their front surface is just visible when skull is 

 viewed from above. Molars* relatively smaller than in the 

 European species of true 3Iicrotus, but the general character of 

 their enamel folding without special peculiarity other than a 

 slight tendency toward roundness in outline of closed triangles of 

 maxillary teeth, particularly the triangles of inner side. The 

 pattern in m^, iv? and m.^^ shows no peculiarities worthy of special 

 note ; m^ without postero-internal loop. Third lower molar with 

 re-entrant angles on oviter side tending to be deeper than usual, 

 * Fig. 149, p. 718. 



