540 



RODENTIA 



Mus viusculus, but brain-case more squarish in outline, rostrum 

 slightly less elongate, and dorsal profile convex throughout, more 

 strongly so posteriorly than anteriorly. Interpariental narrow, 

 the lateral extremities pointed. Auditory bullae rather large, well 

 inflated, their greatest transverse diameter along squarely truncate 

 anterior border. Interorbital region much broader than rostrum, 

 nearly flat, its edges faintly angular. Zygoma abruptly spreading 

 anteriorly, much deeper in front than behind, its lower edge 

 horizontal and at level of alveolar line ; anteorbital foramen large, 

 much wider below than above. Rostrum sloping anteriorly, but 

 rather narrow, the least depth just behind incisors decidedly 

 greater than width at same region ; nasals convex anteriorly, flat 

 posteriorly, the posterior border squarely truncate at level of 

 middle of lachrymal, the nasal branches of premaxillaries slightly 

 longer. Incisive foramina very large, extending from about 

 1 mm. behind alveolus of incisor to level of front of m 1 , the 

 width of each foramen slightly greater posteriorly than anteriorly. 

 Palate rather large, flat throughout, extending at full width 

 conspicuously behind last molars, rather more than half of its 

 surface formed by the palatine bones ; a noticeable transverse 

 ridge at level of middle of ??i 3 , and high narrow longitudinal 

 ridge extending from this to posterior border of palate ; 

 mesopterygoid space about one-half longer than broad, nearly 

 parallel sided, the hamulars slightly bowed outward and coming 

 in contact with antero-internal angle of bullae ; ectopterygoid well 

 developed, forming a conspicuous, nearly horizontal plate, the 

 rather large pterygoid fossa, therefore, shallow. Mandible slender, 

 with no special peculiarities, the well developed coronoid process 

 rising a little above level of condyle ; angular process long, 

 concave on inner side, the lower border bent abruptly upward 

 and outward at nearly a right angle. 



Teetli. — Upper premolar single-rooted, the crown terete, with 

 high, narrow antero-external cusp, and low ridge-like internal 

 cusp, the three cusps separated by two evident furrows. Its area 

 is about one-third that of succeeding tooth. Eirst upper molar 

 3-rooted, the general outline of crown squarish, slightly longer 

 than wide and slightly wider externally than internally ; enamel 

 of crown thrown into four main cusp-like folds, the outer of which 

 are larger than the inner, and three secondary folds extending 

 outward between and alternating with outer cusps. Second molar 

 like first but slightly longer and a little narrower in proportion 

 to its width. Third molar slightly larger than premolar, and of 

 about the same shape, the two anterior cusps small but evident, 

 the two posterior scarcely more than slight elevations on rim 

 of tooth. Lower molars* essentially like the upper teeth, but 

 crowns longer and less squarish in outline, and pattern of folding 

 reversed, the intermediate folds extending inward instead of out- 

 ward. Third lower molar larger than the corresponding upper 

 tooth. 



