1899.] 



NATURAL SCIENCES OF PHILADELPHIA. 



283 



bullse appears to be less iu the females, since I find it jiossible to 

 select the majority of the female skulls iu the series by reference to 

 this character alone. 



Average measurements of five fully adult skulls: greatest 

 length, 29 (28.4-30); basal length, 27.9 (27.6-28.8); basilar 

 length, 26.7 (26-27.4); zygomatic breadth, 18.5 (18-19); mas- 

 toid breadth, 15.2 (15-15.6); interorbital constriction, 4; nasals, 

 7.7 (7.4-8); diastema, 9.3 (9-9.6); palatal length, 15.7 (15- 

 16); occipital depth (in median line), 8.1 (7.6-8.6); fronto- 

 palatal depth (at middle of molar series), 9.1 (9-9.4); mandible, 

 18.5 (18-19); maxillary toothrow (alveoli), 7; mandibular tooth- 

 row (alveoli), 7. 



Teeth. — The enamel pattern is remarkably constant. The last 



loop of the posterior upper tooth is occasionally somewhat more 



abruptly curved than usual on the outer 



Oi^l^v^;^ side so that the last salient angle is cut 



v^ V( M nJ i)ff as a rudimentary closed triangle. 



The variation necessary to bring about 



this result is, however, very trifling. 



In the front lower molar the exact 



Fig. 1. Enamel pattern of form of the anterior loop varies slightly, 



Microius hhjthi. (X 6.) i^,^^ i„ ^^^^ ^f ^^e twenty specimens Ys 



a fourth triangle isolated. 



Remarks. — This species is very closely related to Microtm dmnchi 

 Biichner. A specimen of the latter from northern Thibet agrees 

 with 31. bhjthl in all particulars except its conspicuously shorter 

 tail. 



Measurements. — The measurements of nineteen specimens of 

 Microtus blythi are given in the following table : 



