1918] Kellogg: Microtus calif ornicus Group of Meadow Mice 7 



cisive foramina with sides nearly parallel, and with median portion 

 slightly if at all wider than at ends. 



Zygomatic arches divergent anteriorly, with median portion par- 

 allel to main axis of skull. Jugal broad anteriorly, not deeply em- 

 bedded in maxillary root of zygoma, and relatively short. Zygomatic 

 arches slightly notched at anterior junction with premaxillae. 



Mandible light, with a characteristic convex inferior profile and 

 diastema long. Ridge for masseteres laterales present but not prom- 

 inent. Coronoid process slender, its base narrow, its extremity rising 

 above level of condyle, and strongly curved backward. Angular pro- 

 cess well developed though rather slender, and curved very strongly 

 outward. . The peripheral tuberosity over base of incisor root well 

 developed but not conspicuous. Base of mandibular foramen on a 

 level with, or sometimes above, cutting surface of last lower molar. 

 Mental foramen on lateral face situated slightly below superior sur- 

 face of diastema. 



Teeth. — Molars moderately large relatively to size of skull, their 

 enamel pattern well defined, with deep salient and reentrant angles; 

 upper teeth with noticeable contrasts in lengths of crowns ; M^ much 

 longer than either of the succeeding molariform teeth. First upper 

 molar normally with an anterior transverse loop and four alternating 

 closed triangles; two reentrant angles are normally present on each 

 side ; these are sharply pointed and defined, their points extending 

 beyond median line of crown, those of outer side narrower and deeper 

 than those of inner side ; anterior loop flattened in outline, its inner 

 limb longer and narrower than outer limb, its length less than greatest 

 width of tooth, its main axis oblique to that of tooth row ; inner and 

 outer triangles essentially alike in form, those of inner side slightly 

 the larger. 



Second upper molar with an anterior transverse loop, two outer 

 and one inner closed triangles, rarely the rudiments of a postero- 

 internal loop and two outer and two inner reentrant angles ; anterior 

 loop much like that of M^, but crescentic and wider internally than 

 externally, its long axis nearly transverse to tooth row and if oblique, 

 in reverse direction to long axis of transverse loop of M-, its length 

 nearly equal to greatest width of tooth ; closed triangles similar to last 

 three of preceding tooth. 



Third upper molar with anterior loop and three closed triangles 

 essentially like those of M-, except that closed triangles are smaller 

 and transverse loop is nearly symmetrical and fully as wide as greatest 

 width of tooth ; behind closed triangles lies a long terminal loop sub- 

 tended externally by a slight notch and internally by a deep reentrant 

 angle; this loop in its simplest form is indented at the middle b^^ a 

 deep reentrant angle which imparts to it a strongly curved crescentic 

 outline, the anterior limb longer and more pointed than the posterior 

 limb ; outer convex surface usually marked by a slight salient angle 

 just behind point of attachment to second outer closed triangle ; the 

 posterior limb of crescent is a narrow, rounded, backward-projecting, 

 simple loop, the inner side of which sometimes bears an incipient 

 reentrant angle. 



First lower molar with posterior transverse loop, three inner and 

 two outer closed triangles, and a large anterior loop indented by deep 



