18 BULLETIN 57, UNITED STATES NATIONAL MUSEUM. 



horizontal, their substance thickened and strengthened to receive the 

 articulation of the clavicles, and their combined length eq.ual to 

 or greater than longitudinal diameter of presternum. Mesosternum 

 with a longitudinal median ridge, occasionally rising to a distinct 

 keel, the segments of the bone always fused in adults, and their bound- 

 aries usually obliterated. In cross section the depth of this part 

 of the sternum is usually greater than the width, but to this rule 

 there are conspicuous exceptions. Xiphisternum short, tapering or 

 somewhat expanded posteriorly, its length usually greater than its 

 width, its terminal cartilage well developed. The distinction be- 

 tween the presternum and mesosternum is usually evident, but that 

 between the mesosternum and xiphisternum seldom persists except 

 in the Megachiroptera. 



Shoulder girdle. — The scapula is large, oval in form, the post- 

 scapular fossa much larger than the anterior fossa, its surface divided 

 into three secondary surfaces set at slight angles with each other. 

 The spine is short and moderately high, with a large, strong acro- 

 mion. Coracoid large, usually curved outward, but occasionally 

 straight and directed inward; rarely bifid at tip. Clavicle curved, 

 its length about equal to that of scapula or of longest ribs, its shaft 

 somewhat compressed, in one genus (Diclidurus) (Plate XII, fig. 3) 

 conspicuously expanded. The articulation of the clavicle with the 

 enlarged horizontal anterior lobe of the presternum is by a broad, 

 unusually definite surface, from which the clavicles project upward 

 and outward over the thorax nearly at right angles with each other. 



Ribs. — The ribs decrease gradually in length from the longest to 

 the second; between the second and the unusually shortened, thick- 

 ened first, the reduction is much more noticeable, pr'oducing a break 

 in the otherwise uniform series. Both portions of the first rib are 

 shortened, but the modification, as compared with the others, is most 

 noticeable in the sternal part, which is usually much expanded later- 

 ally. The vertebra to which this rib is attached is usually free, both 

 anteriorly and posteriorly, but not infrequently it becomes ' fused 

 with the last cervical vertebra, even when, as in the Molossidae, no 

 special modification of the shoulder girdle has taken place. These 

 two vertebree, together with the first rib, are so intimately associated 

 with the shoulder girdle in the changes which it undergoes that they 

 may conveniently be treated as forming part of it. 



Modifications of the shoulder girdle and sternum. — The modifica- 

 tions presented by the shoulder girdle and sternum of bats are, as 

 might be anticipated, mostly connected with the mechanical problems 

 of furnishing surfaces of attachment for the very large pectoral 



"For the homologies of the elements included in this region see Leche, Bihang 

 Svensk. Akad Handl., V, No. 15 ; also, Flower, Introduction to the Osteology of 

 the Mammalia, 1885, p. 253. 



