28 BULLETIN 57, UNITED STATES NATIONAL MUSEUM. 
invariably the outer that disappears first. This tooth may be seen in 
every stage of degeneration among the genera with 2-2 upper incisors, 
while in none is it distinctly more developed than its fellow, though in 
some of the long-tongued Pteropidae and in certain genera of Gloss- 
ophaginae it is slightly the larger. 
In the lower jaw the incisors become reduced from the outer side, 
a process mostly associated with narrowing of the anterior portion 
of the mandible. This process is especially well illustrated in the 
Molossidae. The third incisor exists in some species of Nyctinomus 
and in Mormopterus , though reduced in size and crowded beneath the 
cingulum of the canine, the prominent cusp of which has assumed its 
function. In Clioer ephon, Eumops , Promops , and most species of 
Nyctinomus it has disappeared. In some species of Molossops the 
narrowing of the mandible has reduced and distorted the second 
incisor, the cingulum cusp of the canine in turn taking the functions 
of this tooth, while in other species of Molossops , in Eomops , Cheiro- 
meles , and Molossus the second incisor has disappeared. There is 
little doubt that a similar course has been followed throughout the 
Microchiroptera. In the Megachiroptera no genera are known with 
3-3 lower incisors, and it has been assumed by Winge that the first 
tooth is absent, a conclusion based chiefly on the correspondence in 
position of these teeth with the second and third of the upper jaw. 
The probability of this view seems heightened by the almost univer- 
sally larger size of the outer tooth as compared with the inner, while 
in the Microchiroptera the reverse is normally the case. In the fruit- 
eating Phyllostomidae, however, there is a similar reduction in the 
size of the inner incisor as compared with the outer, probably due to 
the action of the tongue, and I prefer to assume that the frugivorous 
habits of the Pteropidae account for the relative size of these teeth 
also, and that the course of reduction in this group forms no exception 
to the rule, so far at least as regards the disappearance of the outer 
tooth first. The next incisor to disappear is, however, probably i 17 
acted upon, as it must have been, by the tongue. In the genera 
Dobsonia , N esonycteris , and Notopteris i 2 is therefore the remaining 
tooth. 
Premolars. — Both above and below it is probably the first pre- 
molar that is permanently absent, though of this there is no proof. 
In the upper jaw the next to disappear is either pm 2 or pm 3 . 
Three premolars are present in members of the families Pteropidae, 
Phyllostomidae, Natalidae, Thyropteridae, Myzopodidae, and Vesper - 
tilionidae. The anterior tooth ( pm 2 ) is the more reduced in four 
of these, the Pteropidae, Phyllostomidae, Natalidae, and Thyrop- 
teridae, while the median ( pm 3 ) is the smaller in the two others, 
the Myzopodidae and Vespertilionidae. While the evidence is there- 
fore not conclusive for any of the other families, it appears safe to 
