1922] Kellogg: Synopsis of Microtus Mordax 291 



Basioccipital in most of skulls examined possesses a poorly defined 

 median ridge. Width of basioccipital at suture between it and basi- 

 sphenoid equal to or less than one-third its median length. Auditory 

 bullae moderate in size, smaller than in mariposae, and considerably 

 inflated; differs from the latter in that posterior margin of bulla is 

 not so strongly indented by foramen, and superior rim of meatus is 

 shorter and not so flaring. 



Palate narrower, more depressed, and with deeper palatine sulci 

 than in mariposae. Lateral bridges and posterior palatine foramina 

 present but not conspicuous. Lateral pits of palate seldom noticeably 

 fenestrated. Pterygoids and hamular processes normally very nearly 

 parallel. Interpterygoid fossa wider than in Microtus calif ornicus 

 group, with posterior margin of palate concave. Incisive foramina 

 equal to or less than two-thirds distance between base of incisors and 

 anterior border of alveoli of molariform series, wide open and narrow- 

 ing at both ends, but with widest portion usually slightly anterior to 

 middle. 



Zygomatic arches not widely or abruptly spreading, with jugal 

 portion parallel to main axis of skull. Jugal slender, about equal 

 in length to that in mariposae, but shallower. Zygomatic arches may 

 or may not be notched at anterior junction with premaxillaries. 



Mandible very light, long and slender. Ridge for masseteres 

 laterales well developed. Coronoid process slender, its base narrow, 

 its extremity rising to or above level of condyle, but with posterior 

 curvature at tip variable. Angular process well developed though 

 rather slender, shorter than in mariposae, not curved as strongly 

 outward. Articular tuberosity over base of incisor, conspicuous. Base 

 of mandibular foramen on a level with cutting surface of last lower 

 molar. Mental foramen on outer face considerably below superior 

 surface of diastema. 



Teeth. — In structure the teeth do not differ appreciably from those 

 of Microtus mordax mordax, though the enamel pattern in general 

 is somewhat variable, especially in M-. The elements of the enamel 

 pattern of the remainder of the molariforin series are very similar 

 to those in mordax; M^ occasionally lacks the notch posterior to meta- 

 eone (fig. F) ; terminal loop seldom crescentic (fig. H), often with 

 incipient external angle (figs. J, K, M, and N) ; an incipient internal 

 reentrant angle is occasionally formed on posterior limb of loop 

 designated as metastyle (fig. 0) ; hypocone may fuse with paracone 

 (fig. I) ; metacone may (figs.- F, G, I, L, and P) or may not (figs. H, 

 J, K, M, N, and 0) be open inferiorly; internal reentrant angle on 

 terminal loop may be almost wanting (fig. L) or may be deep (fig. J). 



Large anterior loop of Pnij designated as secondary paraconid, 

 rarely forms an external prolongation (fig. R) ; section designated 

 as secondary protoconulid often considerably reduced in size (fig. T) ; 

 internal reentrant angles (figs. Q-T) relatively deeper than those 

 of mordax. 



