THE FAMILIES AND GENEBA 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, SHOULDEB, 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 Pteropidse. 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 Megachiroptera, 

 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 f unctionless. At its distal end the ulna is incomplete ; among 

 the Megachiroptera 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. 



