THE FAMILIES AND GENERA OF BATS. 
13 
Wing. 
The general structure of the wing in bats has been so frequently 
described that no detailed account is necessary here. Two special 
parts of the flying apparatus need, however, a few words. 
HUMERUS, SHOULDER, AND ELBOW. 
As might be anticipated from its modification to serve as a wing, 
the anterior limb of the bats differs considerably from that of mam- 
mals that do not fly. The most obvious peculiarity is the great length- 
ening of the fingers to support the flying membrane, but the long 
bones and their two principal joints have also undergone considerable 
changes. The humerus, though in general without special modifica- 
tions in form or unusual development of ridges for muscular attach- 
ment, is peculiar in the large size of the trochiter (tuberculum majus) 
and trochin (tuberculum minus), the former in some families being so 
greatly developed as to extend beyond the head and form a definite 
secondary articulation with the scapula, thus giving the shoulder 
joint a very unusual strength, while limiting its motion strictly to a 
single plane. The bicipital groove is usually deep and well defined, 
especially in the larger Pteropidee. Shaft slender, mostly subterete, 
though somewhat flattened distally, varying in form from distinctly 
sigmoid in the lower groups to nearly straight in the higher, its 
deltoid crest always present, low and broad in the Megacliiroptera, 
high and flangelike in the Microchiroptera. Except for this crest 
the shaft is never marked by distinct ridges, though a faint trace of 
the supinator ridge is sometimes present. No supracondylar fora- 
men or supratrochlear perforation. Internal condyle usually large, 
though in the most specialized groups reduced to a mere base for 
the long spinous process. External condyle obliterated by the dis- 
placement outward of the articular surface. Capitellum large, with 
a well-developed external ridge and groove; trochlea reduced to a 
mere ridge scarcely more prominent than that on outer side of capi- 
tellum. The whole distal articular surface is in most bats so dis- 
placed outward as to be partly or in extreme cases almost wholly 
beyond axis of shaft. The elbow joint is formed almost exclusively 
by the humerus and radius, since the ulna is so reduced as to be 
nearly functionless. At its distal end the ulna is incomplete ; among 
the Megacliiroptera it is continued as a threadlike cartilaginous 
strand to wrist, though in old age.it may become fused with radius; 
in the Microchiroptera this fusion with radius is invariable, though 
the point at which it takes place is not always the same. Occasion- 
ally the proximal extremity also is anchylosed. Olecranon scarcely 
extending beyond radius and forming no definite part of joint, suc- 
ceeded by a sesamoid ossicle nearly equaling it in size. 
