AMEEICAN BATS OF THE GEKEEA MYOTIS AND PIZONYX 7 



general outline of the skull may be narrower {evofis, fig. 7, keenii^ 

 fig. 13) or broader {velifer^ fig. 6, occuUus, fig. 8) ; that the brain case 

 may be high {volans, fig. 10) or low {suhulatus, fig. 11) ; that the 

 area of the rostrum as compared with that of the brain case may be 

 larger {occultus^ fig. 8) or smaller {albescens, fig. 9) ; that the breadth 

 of the rostrum across the roots of the canines may be more {occultus) 

 or less {albescens) than the interorbital constriction; and that the 

 sagittal crest may be conspicuous {velifer, figs. 4, 6) or essentially 

 absent {Tucifugus^ fig. 12). Other important characters which can 

 be less successfully illustrated by photographs are : The greater or less 

 breadth of the palate, including the tooth rows (a measurement best 

 taken at the point of contact between the outer borders of rnr and 

 ni^) as compared with the length of the upper tooth row (canine to 

 third molar, both inclusive), and the greater or less crown area of 

 the upper molariform teeth as compared with the area of the bony 

 palate between the tooth rows {chilo'ensis and nigricans^ fig. 2; 

 velifer and lucifugus, fig. 3). 



Teeth. — The teeth vary in their general size relatively to the size 

 of the skull; but a character of this kind is impossible to describe 

 with any degree of accuracy. The difference in this respect between 

 smaller toothed animals like Myotis ludfugus and M. nigricans., and 

 larger toothed animals like M. velifer and M. chUoensis is, however, 

 appreciable on direct comparison (pi. 1, figs. 2 and 3). 



The incisors and canines, both upper and lower, and the lower 

 molars vary slightly in form; but we have not found that their 

 peculiarities give much aid in the discrimination of species. No 

 American Myotis has yet been discovered in which the second tri- 

 angle of nu is so much reduced as it is in the type of the genus, the 

 palearctic M. myotis. 



The development and relative position of the two minute premolars 

 both above and below is variable individually as well as in different 

 species. Miller has published figures of the upper small premolars of 

 four specimens of Myotis thysanodes,, a species which appears to be 

 especially subject to variation in this respect.^ Usually, there is 

 either a slight crowding, or else the posterior of the small teeth may 

 be drawn inward from the tooth row, particularly in the case of the 

 upper jaw, a condition well developed in the peculiar South American 

 Myotis simus. The most progressive form, as regards the reduction 

 of the tooth-formula, is Myotis occultus. In this animal there is 

 a decided tendency toward the loss of the posterior of the small 

 premolars in the upper jaw, while in the lower jaw the corresponding 

 tooth is usually crowded inward away from the line. Occasional 

 individuals of the other species or races may also lack one of these 

 minute teeth; in most cases it is the second and smaller of the two 



•North Amer. Fauna, No. 13, p. 82, Oct. 16, 1897. 



