Xii IXTUODrCTION. 



ahvays 6 in the family VespertiJiovidse, with the exception of the 

 Californiaii genus Ai-ilrozons, which has here but 4 incisors. In 

 ihis i)articular it shows evidence of its affinity with the family 

 PhyUostomidce, in which 4 incisors in the lower jaw is the normal 

 number. 



The molars are of two kinds : the true molars, and the false or 

 premolars. The former are the larger and situated most pos- 

 teriorly, the latter are small, placed between the true molars and 

 the canines, and appear to unite the characters of both these 

 teeth. The premolar adjoining the first molar bears a stronger 

 resemblance to the grinders than to the premolar adjacent to the 

 canine, which shows decided resemblance to the eye tooth. The 

 number of molars (true and false) in any bat never exceeds 6 

 above and 6 below. In any diminution of this number the first 

 premolar is always wanting. 



The minute description of the teeth is reserved for the remarks 

 under each species. It will be well in this place, however, to de- 

 fine the true molars, and since they are not sulyect to any material 

 variation in shape no mention of them will be made in the text. 



The true molars are 3 in number, both above and below. In 

 the upper jaw they are of a sulj-triangular shape, wider than 

 long, their bases being outward, and their apices rounded and 

 blunt. The first and second teeth have two Y-shaped cusps 

 upon the articulating surface of the crown — the anterior border 

 of each cusp being more prominent than the posterior. The 

 union of these two cusps constitutes what is known as the 

 "W-shaped crown. This irregularity is occasioned by the sinuate 

 incurving of the enamel of the tooth ; it eminently adapts the 

 organ for the mastication of insect food. The inner portion of 

 the articulating face is lower than the outer, is of a rounded 

 shape, and is furnished with but one cusp, which, however, placed 

 immediately behind the anterior triangular cusp, runs obscurely 

 backwards to behind the posterior cusp, giving these teeth the 

 appearance of being qnadri-cuspid. The third molar, much 

 smaller than the preceding, has a straight anterior and a rounded 

 posterior surface ; the external face of crown is irregular and 

 sinuate, posterior unicuspid. 



In the hnoer jaiv the molars are of equal size. They are longer 

 than wide. Each tooth is made up of two Y-shaped cusps, their 



