34 BULLETIN 57, UNITED STATES NATIONAL MUSEUM. 
premolars together. In the genera with large, triangular- crowned 
trenchant premolars (most Phyllostomidse, Natalidse, etc.) the inner 
posterior edge of the upper canine has become knife-like. It is 
crossed by the anterior edge of the anterior lower premolar with a 
true shearing motion, while its cingulum is so formed as to fit the 
point and posterior edge of the smaller tooth. 
Premolars. — In bats with terete premolars the upper and lower 
teeth do not come actually in contact, those of the mandible lying 
distinctly nearer to the sagittal plane than those of the upper 
jaw. The point of pm 2 comes slightly below and behind that of 
pm 3 , while that of pm 4 approaches pm 3 and the cingulum of pm 2 . 
The large upper premolar (^>m 4 ) shears with its interior inner sur- 
face close to but not in contact with the posterior outer surface of 
pm 4 , while its main cusp and posterior cutting edge play against 
the cusp and anterior outer surface of prd 1 °, the extreme point of 
which nearly touches surface of heel. When the premolars are 
trenchant the anterior edge of each lower tooth shears against the 
posterior inner edge of the preceding upper tooth, while the posterior 
edge cuts upon the anterior edge of the corresponding tooth in the 
upper jaw. The relations of the first lower premolar to the upper 
canine have already been explained. 
Molars. — As the distance between the two rows of mandibular 
teeth is sensibly less than that between the opposed sets,^ it follows 
that to obtain a uniformly shearing action of the molars the jaws 
must be thrown either to the right or left before the beginning of 
the stroke. When the mandible is at its extreme lateral position 
the outer and inner cusps of the lower teeth are closely opposed, 
respectively, to the styles and highest cusps of the upper teeth — that 
is, in the outer rows ; prcd 1 to ps 1, lied 1 to ms 1, pred 2 to mts 1 
and ps 2, lied 2 to ms 2, prcd 3 to mts 2 and ps 3, hed 3 to ms 3, in 
the inner rows, ped 1 to the apex of />m 4 , mod 1 to 1, ecd 1 and 
ped 2 to me 1, mod 2 to pc 2, ecd 2 and ped 3 to me 2, med 3 to pc 3, 
ecd 3 to me 3. The triangles of the lower teeth with their concave 
blade-like edges (commissures) tilted slightly inward, and termi- 
nated externally by the recurved cusps face the spaces between the 
similarly concave but outward-tilted edges of the upper triangles, ' 
each lower triangle lying in front of the corresponding triangle of 
« For the sake of brevity the cusps of the three molars may be referred to by 
their abbreviation followed by the numbers 1, 2, and 3. Thus prd l=proto- 
conid of first lower molar ; me 3=metacone of third upper molar. 
6 In a specimen of Noctilio (No. 37435, Mona Island, West Indies), the 
greatest distance between outer edges of upper toothrows is 12.2 mm. ; between 
outer edges of lower toothrows, 8.9 mm. In Myotis calif ornicus (No. 25826. 
San Diego County, California) the same measurements are, respectively, 
4.9 mm. and 2.8 mm. 
