644 CHIROPTERA 
species the ribs are much flattened, and in some they are partially 
ankylosed by their contiguous margins. 
The skull is subject to considerable structural variations, 
even within the limits of a single family. Postorbital processes 
to the frontals are found only in the Péeropodide and some 
Nycteride and Emballonuride. Pteropus leucopterus and Pteralopex 
are peculiar in having the orbit completely surrounded by 
bone. <A slender zygomatic arch is present, except in some of the 
Phyllostomatide. 
The milk-teeth are peculiar in that they are utterly unlike those 
of the permanent series. They are slender, with sharp recurved 
cusps ; and as a rule are shed at an early period (in the Rhino- 
lophide before birth), but may coexist with some of the fully 
developed permanent teeth. The permanent teeth are subject to 
great variation of form, although they always have distinct roots. 
In the Insectivorous types they are acutely cusped, the cusps in 
those of the upper jaw being arranged in a more or less distinct W ; 
but in the frugivorous forms, like the Pteropodide and some of the 
Phyllostomatide, the molars are longitudinally grooved or hollowed 
out. 
The pectoral girdle maintains a very constant type. Thus the 
clavicle is very long, strong, and curved; and the scapula large, 
oval, triangular, with a long curved coracoid process. The humerus, 
though long, is scarcely two-thirds the length of the radius. The 
ulna is rudimentary, its proximal extremity, which articulates with 
but a small part of the humerus, being ankylosed to the radius ; 
and immediately beyond the joint it is reduced to a slender splint- 
like bone, extending about as far as the middle of the radius. In 
all species a detached sesamoid bone exists in the tendon of the 
triceps muscle. The radius is very long, in some species actually 
equal to the length of the head and body. The proximal row of 
the carpus consists of a single bone formed by the united scaphoid, 
lunar, and cuneiform; which, with the extremity of the radius, 
forms the radio-carpal joint. In the distal row the trapezium, 
trapezoid, and magnum vary in size in the different families, the 
unciform appearing to be the most constant, and the pisiform being 
generally very small. 
The manus is always furnished with five digits. The first, 
fourth, and fifth digits consist of a metacarpal and two phalanges ; 
but in the second and third digits the number of phalanges is 
different in certain families. The pollex always terminates in a 
claw, which—like the proximal phalanx—is best developed in the 
frugivorous species. In most of the frugivorous Pteropodide the 
second digit is provided with a claw; but in all other Bats this and 
the remaining digits are unarmed. In the genus 7rianops alone a 
very peculiar short bony process projects from the outer side of 
