THE FAMILIES AND GENERA OF BATS. 
41 
retained by the otherwise more highly modified insectivorous bats 
through a series of stages analogous to those that may now be seen 
in the Phyllostomidse. Owing to the early and complete assumption 
of frugivorous habits these intermediate stages have disappeared 
among the Megachiroptera, and no fossil forms have yet been found 
in which they may be traced. While no genus of Microcliiroptera, 
however strictly frugivorous its members may be, has teeth exactly 
resembling those of the Pteropidse, the change that would be neces- 
sary to pass between such dentitions as those of Pteropus and Phyl- 
lonycteris is much less than those which can be observed step by 
step from the latter back to the normal primitive condition of the 
cusps. 
In its simplest and most characteristic form, as seen, for instance, in 
P ter opus (Plates VII, VIII, fig. 1) this dentition is immediately 
recognizable by the bluntly rounded incisors, large and conspicuously 
ridged canines, and by the striking uniformity of the cheek teeth. 
The first and last cheek tooth both above and below is usually small 
or deciduous, its terete crown flat, concave, or slightly cusped. The 
others have oblong or squarish crowns bearing a large outer and a 
small inner elevation, these elevations high and cusp-like in the 
more anterior teeth, becoming successively lower toward the posterior 
end of the series, where, particularly in the lower jaw, they may be 
scarcely more than the rims of the conspicuous longitudinal median 
groove. The distinction between the inner and outer cusp is not 
always evident, particularly in the first large tooth of the lower jaw, 
and the space between them is usually somewhat filled in by the ridge- 
like inner bases of the cusps. Anteriorly where the cusps are high 
the crushing surface is strongly oblique, while posteriorly, where 
the cusps are low, it is nearly horizontal. While no intermediate 
stages are known directly connecting this type of dentition with 
normally cusped ancestral forms, it is safe to assume, from analogy 
with the frugivorous Phyllostomine bats, that in the upper molars 
the two cusps are the protocone and paracone and in the lower 
molars the protoconid and metaconid. 
Teeth essentially like those of Pteropus are found in many genera 
of Megachiroptera. In Styloctenium and the long-tongued genera 
simplification has taken place, in the first instance by broadening 
and flattening the cusps and ridges until a nearly cushion-shaped 
crown results, in the second (Plates VII, VIII, fig. 4) by reduction 
in the size of both crowns and elevations. The more usual type of 
divergence is seen in the tendency to produce additional cusps and 
ridges not homologous with the parts of the primitive insectivorous 
tooth. This is well illustrated by Nyctymene (Plates VII, VIII, 
fig. 2) and Niadius (Plates VII, VIII, fig. 3). In the former the 
first, second, and third of the large mandibular cheek teeth develop 
