INTERRELATIONSHIPS OF GENERA 51 



rather broad interorbital region, and oblong and depressed brain- 

 case. The temporal ridges are widely separated in the inter- 

 orbital region and behind; the squamosals have moderately 

 salient post-orbital crests and are widely separated anteriorly, 

 although in old age they show a slight tendency to encroach 

 upon the frontals and approach each other. The upper border 

 of each jugal bone is convex, giving a fusiform expansion to the 

 central part of the zygomatic arch. The upper portion of the 

 infraorbital canal is more spacious than usual. The palate 

 posteriorly is formed nearly as in Microtus, but the postero- 

 lateral bridges are usually absent, the postero-median septum is 

 short and horizontal, and the postero-lateral pits are very shallow ; 

 these features of low relief in the palate may be correlated 

 with the brachyodonty of the cheek-teeth, and cause the 

 palate of adult Phenacomys to resemble that of Microtus and 

 other more highly developed voles in young stages of growth. 

 The pterygoid fossise are shallow, their floors being nearly flush 

 with the ventral surface of the basisphenoid. The palate, 

 choanse, basisphenoid, and basioccipital are broad. The auditory 

 bullae are small, globular, simple and without internal spongy 

 tissue ; the stapedial artery is naked as it approaches the stapes. 

 The mandible is nearly normal, but m^ is not displaced by the 

 shaft of the lower incisor, which passes below m^ to terminate 

 in the base and at the hinder margin of the condylar process 

 below the dental foramen; the groove between the alveoli of 

 the cheek-teeth and the ascending ramus is not " pocketed " 

 posteriorly by the alveolar sheath of the lower incisor as in 

 Microtus, but remains open behind as in Evotomys. 



Although some primitive characters are thus to be observed 

 in the skull and mandible of Phenacomys the claim of the genus 

 to a lowly position among voles rests chiefly upon the evidence 

 of the cheek-teeth. These are very light and of limited growth, 

 each molar developing two roots when adult. The enamel 

 shows the beginning of a normal differentiation, being slightly 

 thicker on the concave than on the convex sides of the salient 

 angles. The re-entrant folds are narrow and transversely deep, 

 giving to the teeth a characteristic longitudinally crowded 

 appearance, but they do not contain cement. The enamel 

 pattern is peculiar, characterized especially by the approximate 

 equality of the inner and outer salient angles and infolds in 

 upper molars and by their disparity in lower molars, in which 

 the inner salient angles are conspicuously larger and the inner 

 infolds much deeper than those of the outer side; m^, m^, wig 

 and ?»3 are essentially as in normal voles, the last-named tooth 

 being, however, in living species of Phenacomys, usually a little 

 more reduced than usual, its antero-external angle becoming 

 obsolete ; w^ is also noticeably reduced in living species, having 

 only three salient angles on each side; ?% is a complex tooth 

 with four or five outer and six inner salient angles, consisting 



