1918] Kellogg: Microtus calif ornicus Group of Meadow Mice 33 



orbital ridges, usually well marked. Interorbital ridges variable in 

 appearance, usually rounded, but with well defined sulcus between 

 them. Interparietal variable in outline. In most cases biconvex 

 posteriorly, its sides squarely truncated, but with blunt median pro- 

 jection. 



From a dorsal aspect the rostrum tapers narrowly anteriorly. 

 Least depth behind incisors greater than width in same region. Nasals 

 long, spatulate, widening at a point anterior to middle, their posterior 

 terminations emarginate. Ascending branches of premaxillae exceed- 

 ing posterior terminations of nasals and extending as far as plane of 

 lacrimals. Vertical diameter of anterior narial passage about equal 

 to transverse diameter. Anteorbital foramen may be high and narrow 

 or sbort and broad. 



Occiput flattened, the median depth averaging about 60% of the 

 greatest width across lambdoidal ridge of brain-case. Foramen mag- 

 num subtriangular. Paroccipital processes usually directed more 

 downwards than backwards, and not normally applied to bullae at 

 their extremities. Posterior borders of these processes continued 

 upwards on supra-occipital as sharp crests. Basioccipital shows a 

 well defined median ridge. Width of basioccipital at suture between 

 it and basisphenoid equal to one-third or more of its median length. 

 Auditory bullae large, strongly inflated, their posterior borders ob- 

 liquely truncated, their inner sides rapidly diverging posteriorly. 



Palate has well marked palatine sulci and prominent median ridge. 

 Lateral pits of palate fenestrated. Pterygoids and hamular processes 

 rapidly diverging posteriorly. Posterior margin of palate iisually 

 rounded. Interpterygoid fossa narrow. Posterior palatine foramina 

 distinct. Incisive foramina variable ; usually narrow, slightly ex- 

 panded posterior to middle and rounded at both ends. 



Zygomatic arches rapidly diverging anteriorly, but with posterior 

 portion nearly parallel to main axis of skull. Jugal enlarged, ex- 

 panded centrally, and mortised into maxillary root of zygoma. Supe- 

 rior width of maxillary root of zygoma greater than that in Ivupcr- 

 uthrus. Zygomatic arches conspicuously notched at anterior junction 

 with premaxillae. 



Mandible robust but smaller than in mohavensis. Masseteric 

 ridges well developed. Coronoid process light, its base narrow, its 

 extremity rising above level of condyle and curved backward. Angu- 

 lar process long, curved outward, but not extending posteriorly be- 

 yond plane of condyle. Peripheral tuberosity over base of incisor 

 lying for the most part above masseteric ridge. Base of mandibular 

 foramen above level of cutting surface of last lower molar. Mental 

 foramina on lateral face situated somewhat below superior surface 

 of diastema. 



Teeth. — Essentially like those of californicus except that closed 

 triangles are relatively wider. Anterior loop of M^ usually crescentic, 

 though in some individuals it is flattened anteroexternally. No inter- 

 nal lobe present on posterior triangle of M-, although there is a rudi- 

 mentary posterointernal loop in some specimens. Enamel folding of 

 M- without any especial peculiarity, though long terminal loop is 

 variable in outline. Inner side of posterior limb of terminal loop may 

 bear an incipient reentrant angle. 



