﻿82 



T^^ORTH AMERICAN FAUNA. 



Fig. 16.— Maxillary teeth of four speci- 

 mens of Myotis thysanodes, showing in- 

 dividual variation in form and position 

 of premolars : a, specimen from Patzcu- 

 aro, Michoacan ; b, c, and d, from Ha- 

 cienda La Parada, San Luis Potosi ( x 5) . 



case, forehead more abraptly elevated above muzzle and rounder less 

 angular occiput. When viewed from above, the posterior margin of 

 the brain case is rounded in M. thysanodes, truncate in il/. velifer. When 

 viewed from behind, the brain case in M. thysanodes is broader in pro- 

 portion to its height than in M. velifer and lacks the conspicuous 

 occipital crest of the latter. The posterior part of the palate, from the 

 last molars to the tips of the hamulars, is shorter in proportion to 



the distance between the hindermost 

 molars in ill. velifer than in M. thysanodes. 



Teeth. — Upper incisors diverging at the 

 tips. First and second upx^er premolars 

 very variable in relative position and 

 size. The first is always much longer 

 than the second and has the crown at 

 least one-fourth larger in cross section. 

 The cross section of the first may, how- 

 ever, be nearly twice that of the second. 

 In some specimens these two small pre- 

 molars are perfectly in the line of the 

 tooth row, the first in contact with the 

 canine, the second touching the first, but 

 separated from the third by a distinct 

 space. In others the second premolar 

 while i^erfectly in line is in contact with the third as well as with the 

 first. Earely the second ijremolar lies slightly external to the tooth 

 row, while very commonly it is displaced to a varying degree inward, 

 so much so in some cases as to be almost hidden from the outer side by 

 the close approximation of the first premolar and the anterior edge of 

 the third. These variations are independent of age and sex. The 

 extremes with intermediates of all degrees occur among a dozen of the 

 females collected by Mr. Nelson at Hacienda La Parada, San Luis 

 Potosij August 16, 1892 5 while specimens with teeth much worn or 

 wholly unworn may have the premolars in- 

 differently greatly crowded and displaced or 

 wholly in the tooth row (fig. 15 d^ fig. 16, and 

 fig. 17.) 



Third premolar triangular in outline, the 

 outer border abruptly convex in front, and 

 equal to posterior border; anterior and poste- 

 rior borders concave; inner apex rounded, not extending back to level 

 of inner margins of molars. First and second molars trapeziform, the an 

 terior edge longest, the posterior outer and inner margins successively 

 shorter. Anterior border straight to near inner edge, where it is bent 

 abruptly backward, posterior border very slightly concave. First 

 molar shorter and broader than second, and with anterior border nearly 

 straight. 



Fig. 17. — Ahnormal premolar of 

 Myotis thysanodes (No. 52228) : a, 

 crown ; h, side ( X 20) . 



