1918] Kellogg : Microtus calif ornicus Group of Meadow Mice 35 



interorbital depression. Occiput rounded, concealing the condyles to 

 some extent when skull is viewed from above. 



While there is considerable variation among different individuals, 

 the brain-case is usually subquadrate, the sides rounded, and truncated 

 anteriorly by postorbital tuberosities. Longitudinal ridges along outer 

 edges of parietals, when present, but feebly developed. Interorbital 

 ridges variable but always separated by sulcus of varying width. 

 Interparietal somewhat rectangular, with posterior border convex and 

 with outer extremities rather abruptly truncate. Anteroposterior 

 diameter exclusive of median projection less than one-half, occasion- 

 ally one-third, greatest transverse diameter. Frontals shallowly convex 

 or truncate porteriorly. 



Rostrum rapidly tapering anteriorly. Least depth of rostrum be- 

 hind incisors greater than width in same region. Nasals long, spatu- 

 late, their posterior terminations squarely truncate. Ascending 

 branches of premaxillae exceeding nasals and extending to or beyond 

 plane of lacrimals. Vertical diameter of anterior narial passage less 

 than, or at the most equal to, transverse diameter. Anteorbital fora- 

 men variable in outline, usually considerably wider dorsally than in 

 neglectus. 



Occiput more convex and less flattened than in neglectus, but 

 median depth may equal as much as 58% of greatest width across 

 lambdoidal ridge of brain-case. Foramen magnum subtriangular. 

 Paroccipital processes directed more downwards than backwards, with 

 tips usually not applied to bullae. Posterior margins of paroccipital 

 processes continued upwards on supra-occipital, terminating below 

 lambdoidal ridge in a prominent though blunt crest. 



Floor of brain-case with poorly defined median ridge. Width of 

 basioccipital at suture between it and basisphenoid equal to one-third 

 of its median length. Auditory bullae smaller than those of neglectus, 

 their posterior borders obliquely truncated, their inner sides rapidly 

 diverging posteriorly. 



Palate narrower than in neglectus, with deep though narrow pala- 

 tine sulci. Lateral pits of palate fenestrated. Interpterygoid fossa 

 narrow, the hamular processes not markedly diverging as in above. 

 Posterior palatine foramina distinct. Incisive foramina in most speci- 

 mens narrower than in above, slightly expanded anterior to middle, 

 and narrowing at both ends. Zygomata widely spreading, the upper 

 surfaces of maxillary arms of zygoma narrower than in neglectus. 

 Jugal narrower and less sharply mortised into maxillary root of 

 zygoma. 



Mandible robust, with moderate diastema. Masseteric ridges well 

 developed. Coronoid process long and light, its base narrow, its ex- 

 tremity rising above level of condyle and curved backward. Angular 

 process long, curved outward, but not extending posteriorly to 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 situated 

 on lateral face below superior surface of diastema. 



Teeth. — Molars smaller relatively to size of skull than in neglectus, 

 their enamel pattern well defined, with sharp salient and reentrant 

 angles. Anterior loop of M^ flattened. An internal lobe may or may 



