108 MR. R. F. TOMES ON THE GENUS VAMPYRXJS. [Mar. 26, 



hold about the same relative proportion which is usually observable 

 in the larger and smaller species of any natural group, it cannot be 

 made any proper use of in determining the limits of genera. But 

 the incisors and premolars in the lower jaw exhibit considerable di- 

 versity, not only in regard to number, but in size and relative pro- 

 portion also. 



These peculiarities in the dentition, in connexion with the super- 

 ficial ones above stated, appear to me to supply the most certain 

 means by which to classify the several species ; and I will now pro- 

 ceed to state what I regard as their relationship with each other. 



The normal number of incisors in Phyllostoma is /our in each jaw ; 

 and the number of premolars is two on each side, above and below. 

 In the Vampyrus spectrum this normal number of incisors obtains ; 

 but among the premolars in the lower jaw is an additional one. In 

 V. auritus, having an expansion of membrane not inferior to that 

 of V. spectrum, this supernumerary premolar is of rudimentary form, 

 and though irregular in position, is nevertheless present, and is ac- 

 companied by an abnormal number of inferior incisors, which are 

 reduced to tivo. This species, then, 1 regard as possessing the col- 

 lective characters of Vampxjrus more fully than any other. Of Lo- 

 phostoma sylvicolum I can say but little, not having examined a spe- 

 cimen. It appears to make a somewhat near approach to Vampyrus 

 auritus, excepting that the wings are narrower, and the ears less de- 

 veloped, in which respects it may make some approach to the follow- 

 ing species. Schizostoma appears in general conformation, and in 

 dentition, to come nearest to V. spectrum, but differs in having a tail, 

 and in having the membranes of the wings extend only to the distal 

 extremity of the tibia. The two following so-called genera Mimon 

 and Tylostoma are identical, and intermediate between Vampyrus and 

 Phyllostoma, With the lower incisors of the former they have the 

 lower premolars of the latter, and an intermediate general outline. 

 The last genus, Trachops, has the dentition oi Phyllostoma, and re- 

 sembles Vampyrus in its large oval ears and considerable breadth of 

 alar membrane. 



We have in these species a complete gradation from Vampyrus to 

 Phyllostoma ; and while I admit these as distinct genera, I am scarcely 

 willing to admit the intermediate and allied forms which are here 

 brought together, as even of subgeneric pretensions. This will be 

 better understood by looking at the exact amount of difference be- 

 tween the species most typical of these genera (i. e. Vampyrus auritus 

 and Phyllostoma hastatum) ; and it will be seen that it is merely one 

 o{ degree, of which degree the remaining species constitute the middle 

 portion. 



Perhaps it will seem that sufficient importance has not been attached 

 to the presence or absence of a tail in these species. In nearly all 

 the PhyllostomidcE the tail is of rudimentary proportions, seldom ex- 

 ceeding half the length of the interfemoral membrane. I have 

 usually observed that, when an organ is but feebly represented 

 throughout a group of Mammals, it commonly presents varied de- 

 grees of development in different species in that group, and even in 



