1918] Kellogg: Microtus californicus Group of Meadow Mice 25 



Skull. — In some respects resembles that of vallicola, in others that 

 of aestuarinus. Dorsal profile rather evenly convex, slightly depressed 

 interorbitally. Width across zygomatic arches greater than in any 

 other form within the group. Occiput rather abruptly truncate, 

 concealing the condyles to some extent when skull is viewed from 

 above. Ventral profile similar to that of vallicola. 



Brain-case in outline broadly ovate, rounded at sides and trun- 

 cated anteriorly by poorly developed postorbital tuberosities. In 

 old adults the longitudinal ridges near outer edges of parietals are 

 present and unite with interorbital ridges in frontal region. Inter- 

 orbital ridges variable in appearance but apparently always sepa- 

 rated by distinct sulcus. Interparietal somewhat rectangular, with 

 posterior border convex and with outer extremities rather abruptly 

 truncate. Anteroposterior diameter exclusive of median projection 

 less than one-half greatest transverse diameter. Frontals convex or 

 truncate posteriorly. 



Rostrum rapidly tapering anteriorly ; width at narrowest region 

 considerably greater than breadth of interorbital constriction ; least 

 depth behind incisors greater than width in same region Nasals 

 narrowly spatulate, their posterior terminations squarely truncate or 

 emarginate. Ascending branches of premaxillae exceeding nasals 

 and extending to or beyond plane of lacrimals. Vertical diameter of 

 anterior narial passage equal to transverse. Anteorbital foramen 

 variable in outline, with its superior portion in some individuals 

 scarcely if at all wider than inferior, though superior width is usually 

 greater than inferior. 



Occiput evenly convex or flattened, the median depth equal to 

 about 60% of greatest width across lambdoidal ridge of brain-case. 

 Foramen magnum svibtriangular. Paroccipital processes directed 

 more downwards than backwards, with tips not applied to bullae. 

 Posterior margins of paroccipital processes continued upwards on 

 supra-occipital as sharp crests, terminating at or below lambdoidal 

 ridge. 



Floor of brain-case with sharply defined median ridge. Auditory 

 bullae large, with their posterior borders obliquely truncated. Width 

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

 third its median length. 



Palate relatively wider than that of vallicola, with slightly deeper 

 palatine sulci and narrower median ridge. Interpterygoid fossa nar- 

 row, though hamular processes rapidly diverge posteriorly. Poste- 

 rior palatine foramina small though open. Lateral pits fenestrated. 

 Incisive foramina long, wide open, the outer sides nearly parallel 

 throughout their length, and ends rounded. 



Zygomatic arches diverging rapidly anteriorly, the sides nearly 

 parallel ; upper surface of maxillary root narrower than in vallicola. 

 Jugal somewhat expanded anterioi' to middle, and deeply mortised 

 into maxillary root of zygoma. Zygomatic arches deeply notched at 

 anterior junction with premaxillae. 



Mandible robust, with moderate diastema. Ridge for masseteres 

 laterales well developed. Coronoid process heavy, its base wide, its 

 extremity rising to or above level of condyle, though but little curved 

 backwards. Angular process heavy, well developed, curved outward, 



