40 MICROTIN^ 



In Myopus the external form is also vole-like, although more 

 thickset than in Synaptomys. The skull and teeth agree in essen- 

 tial respects with those of Lemmus, although the skull is dis- 

 tinguished from that of Lemmus by its smaller size, lighter 

 build, rather less expanded zygomata, anteriorly more widely 

 separated squamosals, larger anterior palatal foramina, and more 

 globular auditory bullse. In these various characters the genus 

 shows itself to be more specialized than Synaptomys, although 

 outwardly, and to some extent in cranial characters, more 

 primitive than Lemmus. 



In Lembius the characters of the group reach their highest 

 expression, the whole animal being greatly modified for fossorial 

 habits. Outwardly this is shown by the extremely robust general 

 form, small eyes, small ears hidden in the fur and destitute 

 of meatal valves, large and broad hands and feet, enlarged 

 fore-claws, peculiar large and flattened thumbnail, hairy palms 

 and soles, with the pads reduced to functionless vestiges, and 

 short, thick, and densely clothed tail. Under the influence of 

 the powerful temporal muscles and of fossorial habits the skull 

 has become massive, broad, and depressed, with very strongly 

 built, abruptly and widely expanded zygomata; in the inter- 

 orbital region the temporal ridges fuse to form a median crest; 

 the squamosals form strong post-orbital crests and tend to 

 approach each other anteriorly as age advances ; the braincase is 

 square, its post-glenoid portion much shortened, and it seems to 

 have been pushed forwards, encroaching upon and reducing 

 the size of the temporal portions of the orbito-temporal vacuities ; 

 the cheek-tooth rows diverge rapidly behind ; the palate termin- 

 ates simply behind, the posterior median sloping septum being 

 represented merely by a short, free, spinous process ; the ptery- 

 goid fossEe are very short and deep. The auditory bullae are less 

 inflated than in the related genera, but the cavity of the middle 

 ear is filled with a dense sponge of bone. The incisor teeth are 

 rather slender, the upper ones rather strongly curved and with- 

 out well-marked anterior grooves. The cheek-teeth are broad 

 and heavy, with the salient angles on the inner sides of the upper 

 molars and the outer sides of the lower molars squarely truncated. 



The lowest Microti retain many primitive characters not 

 found among the highly specialized genera which alone are known 

 to represent the Lemmi. But in one important respect the most 

 primitive voles stand upon a higher plane than any of the 

 lemmings. The lower incisor in all voles has pushed its way back- 

 wards through the jaw to a point considerably behind mg, a 

 character which indicates much later divergence from the primi- 

 tive Murine stock by Microti than by Lemmi. The shaft of the 

 lower incisor, lingual to the molars anteriorly (as it is through- 

 out its course in the lemmings), crosses obliquely below or be- 

 tween the roots of m2 and m,^ to the labial side of the jaw, where 

 the growing base of the tooth invades and finally, in the most 



