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



Floor of brain-case similar to that in calif ornicus. Basioccipital 

 with an illy defined median ridge. Width of basioccipital at suture 

 between it and baSisphenoid more than one-third its median length. 

 Auditory bullae large, somewhat inflated, gradually sloping from 

 inner margin to outer, and with region below meatus slightly flattened. 



Palate with rather deep palatine sulci and somewhat elevated 

 median ridge. Lateral bridges of palate well developed, but lateral 

 pits shallow and usually fenestrated. Pterygoids and hamular pro- 

 cesses diverging rapidly posteriorly. Interpterygoid fossa relatively 

 narrow, but with posterior margin of palate usually concave. Poste- 

 rior palatine foramina of moderate size. Incisive foramina long, nar- 

 row and wide open, somewhat wider posterior to middle than at ends ; 

 the latter rounded. 



Zygomata broadly spreading posteriorly, the outer border passing 

 forward in an evenly convex curve to anteorbital foramen, with rela- 

 tively heavy maxillary arm. Jugals with well marked central expan- 

 sions, their upper borders boldly convex, and with their heads deeply 

 mortised into maxillary roots of arches. The zygomatic arches are 

 somewhat notched at anterior junctions with premaxillae. 



Mandible robust and heavier than in californicus. Ridge for 

 masseteres laterales well developed. Coronoid process large, its base 

 broad, its extremity rising to level of condyle and distinctly curved 

 backward. Angular process well developed though rather heavy and- 

 curved strongly outward. The peripheral tuberosity over base of root 

 of incisor present but not conspicuous. Base of mandibular foramen 

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

 foramen situated below superior surface of diastema. 



Teeth. — While preserving the typical californicus pattern, the 

 teeth are peculiar in a certain quadrangular shape of the triangles. 

 Anterior loop of M^ crescentic. An internal lobe usually present on 

 posterior triangle of M-. A small proportion of individuals have this 

 posterior lobe entirely closed, but it is usually open. Long terminal 

 loop of M^ variable in outline, usually crescentic but sometimes 

 strongly indented externally by a notch. Internally the loop is 

 notched by a deep reentrant angle. 



Lower molars not differing in any essential respect from those in 

 other members of the group excepting that posterior transverse loops 

 are proportionally wider. 



Remarks. — I am unable to agree with Bailey's statement (1900, 

 p. 38) that since californicus and edax {== aestuarinus) differ so 

 widely in habits and distinctive characters, and also since their ranges 

 conform to different zones, there is sufficient reason to recognize them 

 as full species. Specimens from the tidal marshes a short distance 

 south of Petaluma, Sonoma County (shown especially by skull no. 

 3659) indicate that eximius intergrades with aestuarinus in this 

 region. If cranial characters alone were used, some skulls from this 

 locality could be referred to one race as readily as to the other. The 

 individuals which inhabit the tidal marshes are much blacker than 



