1918] Kellogg: Microtus calif ornicus Group of Meadow Mice 11 



slightly short of terminus of premaxillae. Opening for anterior nares 

 higher than wide in adults. Anteorbital foramen relatively narrower 

 and shorter as compared with that of eximius. 



Occiput slightly flattened, but median depth may equal as much 

 as 60% of greatest width across lambdoidal ridge, though the latter 

 scarcely rises to level of occiput. Foramen magnum roughly tri- 

 angular, but with angles more flattened. Paroccipital processes rela- 

 tively heavy, directed more downwards than backwards, their poste- 

 rior margins continued upwards as crests, as usual, but lighter. Basi- 

 occipital with a faint or rudimentary median ridge. Bullae relatively 

 small and narrow, somewhat inflated, with region below meatus some- 

 what depressed. 



Palate rather broad, with shallow palatine sulci and somewhat 

 widened median ridge. Lateral bridges fairly well developed. Lateral 

 pits of palate shallow. Pterygoids and hamular processes diverging 

 more widely posteriorly. Interpterygoid fossa narrower than in 

 eximius and occasionally with posterior palatal margin notched. 

 Posterior palatine foramina present but not distinct. Incisive fora- 

 mina long and wide open, with sides slightly expanded. 



Zygoma with squamosal root very light, upper surface of maxillary 

 root narrow, and with median portion parallel with main axis of skull. 

 Jugal short and slender. Zygomatic arches not notched to any 

 appreciable extent at anterior junction with premaxillae. 



Mandible small and relatively slender ; ridge for masseteres later- 

 ales fairly well developed. Coronoid process small, its base narrow, its 

 extremity rising above level of condyle and slightly curved backward 

 at tip. Angular process light and curved outward. The peripheral 

 tuberosity over base of incisor barely visible. Base of mandibular 

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

 Mental foramen on lateral face situated somewhat below superior 

 surface of diastema. 



Teeth. — Not differing appreciably in structure from those of cali- 

 f ornicus, though molariform series are much smaller. Anterior loop 

 of M.- flattened. An internal lobe may or may not be present on 

 posterior triangle of M-. Enamel folding of M- without any especial 

 peculiarity excepting that long terminal loop is more strongly sub- 

 tended externally by notch, and outline is much less concentric than 

 in eximius. 



Mj; and M^ similar to those of calif ornicus, but M-, with only three 

 loops, whereas in caMfornicus there may be an appearance of four 

 due to constriction of the second loop to form two triangles. Trans- 

 verse loops relatively wider. 



Remarks. — This is a small race of the calif amicus group, with a 

 notably restricted range. There is a peculiar parallel, as pointed 

 out by Bailey (1900, p. 37), in the characters presented by Microtus 

 mordax angusticeps and Microtus californicus constrictus. In both 

 there are similar modifications in cranial characters which separate 

 them from their nearby related races. They represent depauperate 

 coastal forms of two widely distributed inland species. 



