124 BULLETIN 144, UNITED STATES NATIONAL MUSEUM 



larger and more robust. As compared with the skull of Myotis veli- 

 fer, with which it agrees in general size, that of M. thysaTwdes is 

 more slender and delicate, the brain case narrower and more oval in 

 outline. The sagittal crest is about as well developed as in M. velifer 

 but the occiput is inflated posteriorly so that the lambdoid crests do 

 not meet at the vertex to cut off a raised triangular area. As seen 

 from above, therefore, the hind margin of the skull is not truncate 

 but is bowed backward ; while in rear view the vertex is not peaked 

 but is evenly arched. In palatal aspect the distance between the last 

 molar and the audital bulla is obviously less, both relatively and 

 absolutely than in Myotis velifer; the palate ends in a well-defined a 

 projection medially. 



Teeth. — The molars are normal in the relation of their width to 

 that of the palate, not enlarged as in Myotis velifer and M. occultus^ 

 though distinctly larger than in M. evotis. In the first and second 

 tooth there is no metaloph and the protoconule and paraloph are 

 usually absent (fig. 1 e, p. 8), though either or both may be slightly 

 developed, particularly in m^. This extreme simplification of struc- 

 ture is not known in other American species of Myotis^ though it is 

 occasionally approached in M. evotis. The third molar, as in all the 

 other known American members of the genus, retains a distinct 

 protoconule. Cingulum of m} and 7n? usually wide and distinct, 

 better developed than in Myotis velifer, though not actually passing 

 around the antero-lingual base of the protocone. At the postero- 

 lingual base of the hypocone the cingulum occasionally becomes 

 thickened and tubercle-like, a condition especially well shown in the 



type. 



The two small upper premolars are usually both visible in side 

 view, though drawn inward slightly from the tooth row; there is 

 somewhat more variation in their exact position than in related 

 species. In one specimen (52186 U.S.N.M.) both the minute upper 

 premolars are missing on the left side while on the right side the 

 larger only is present, almost completely filling the space between 

 canine and p*, while in another (62228 U.S.N.M.) the two small 

 teeth are united." 



Remarks. — Externally Myotis thysaxnodes may be distinguished 

 from M. evotis by its greater size and less enlarged ears. Its fur 

 is not so full nor so golden in color, a difference fairly obvious on 

 direct comparison. The ears are usually brownish instead of black. 

 The free edge of the tail membrane is so thickly edged with short 

 coarse hairs that they form a fringe which is clearly obvious to 

 the naked eye, whereas in Myotis evotis the fringe is less dense 

 and not distinctly visible without a lens. The presence of this 



" Figured by Miller, North Amer. Fauna, No. 13, p. 82, Oct. 16, 1897. 



