1894.] Zoology. 183 
Two triangles of equal size, their bases formed by the median line of 
enamel connecting the bases of the three larger opposing internal tri- 
Fi 
AULACOMYS ARVICOLOIDES, Type. 
Explanation of Plate.—1. Skull, from above. 2. Same, from below. 3. Exterior 
of left Mandible. 4. Anterior view of superior Incisors. 5. Crown of right, lower 
first Molar. (Figs. 1, 2, 3 & 4 x 134 diameters; fig. 5 x 214 diameters). 
of base of last inner triangle. The molar series are prismatic and non- 
rooted, Remainder of molar dentition much as in the genus Arvicola. 
Owing, however, to the greater relative depth and width of the entrant 
angles in Aulacomys the basal corners of opposing triangles of the 
lower molar series do not overlap as in Arvicola but stand distinctly 
upon their respective sections of the median enamel wall. Frontal 
bones flattened superiorly and lacking trace of supraorbital ridges. 
Nasals, short, abruptly triangular, terminating posteriorly in a point, 
very broad anteriorly and deeply notched subterminally. Nasal pro- 
cess of premaxillary, reaching behind anterior plane of orbits, far be- 
hind base of nasals and terminating in a slender point. Auditory bul- 
lae, triangular, narrow, not encroaching on basisphenoid, the tympanic 
process of the meatus (viewed from below) lying within lateral profile 
of the brain case. Postpalatal notch acute, terminating the hastate 
pterygoid fossa, so formed by the contraction of the pterygoids. Con- 
