New Species of Aplodontia. 319 



difference in the size of the skulls. Hence it appears that the 

 ratio of the average length of this portion of the premaxillary to 

 the average of the basilar-length, in four skulls of A. major, is 

 36.12 ; while in four skulls of A. rufa it is 39.00. 



In all of the skulls of A. major excepting the youngest (No. 

 2103 Mus. C. H. M.) the nasals terminate posteriorly either ex- 

 actly or nearly on a line with the premaxillaries. In the young 

 male No. 2103, they fall short of this line by 2 mm. The con- 

 trary is true in the series of eight skulls of A. rufa. Here the 

 nasals normally fall short of the fronto-premaxillary suture by 

 from 1 to 5 mm., in exceptional cases only ending flush with it. 

 Still, since both conditions are found in both species, this char- 

 acter cannot be regarded as distinctive. 



But the nasal bones furnish another character apparently more 

 constant than the foregoing. In adult males of A, major, the 

 greatest breadth of the nasals is at or near their anterior extremi- 

 ties, while in A. rufa it is situated some distance posteriorly — 

 usually about 8 or 10 mm. from the anterior end of the bone. In 

 the specimens before me, this difference is very marked, and the 

 greatest breadth in A. rufa is absolutely as well as relatively 

 greater than the measurement of the corresponding part in A. 

 major. For example, in the largest skull of A. rufa (No. 11358 

 U. S. Nat. Mus.) the nasals anteriorly measure 11.90 mm. At 

 a point 10 mm. posteriorly their breadth is 13.40 mm. In the 

 largest male of A. major (No. 2106 Mus. C. H. M.) these mea- 

 surements are respectively 12.60 mm. and 12 mm. Hence at a 

 point 10 mm. from the front end of the nasals, the breadth of 

 those of A. rufa is actually 1.40 mm. greater than those of A. 

 major, while the skull of the latter is much the larger. 



In brief, skulls of A. major are distinguishable from those of 

 A. rufa (1) by absolute size ; (2) by absolute weight ; (3) by the 

 ratio of the upper molar series to the basilar-length ; (4) by the 

 contour of the occiput and the development of the occipital crest; 

 (5) by the distance between the antorbital foramina ; (6) by the 

 shape of the zygomatic arches and of the vacuities which they 

 enclose ; (?) by the shape of the nasal bones, and by the ratio of 

 their length to that of the ascending process of the premaxillary; 

 (8) by the ratio of the ascending process of the premaxillary to 

 the basilar-length ; (9) by the distance between the antorbital 

 foramina; and (10) by the fronto-parietal length. 



