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 Microchiroptera, 

 however strictly frugivorous its members may be, has teeth exactly 

 resembling those of the Pteropidae, 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 

 Pteropus (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 



