18 BULLETIN 57, UNITED STATES NATIONAL MUSEUM. 
horizontal, their substance thickened and strengthened to receive the - 
articulation of the clavicles,® and their combined length equal 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, producing 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 Molossidse, 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. 
M odifications 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 Hand!., V, No. 15 ; also, Flower, Introduction to the Osteology of 
the Mammalia, 1885, p. 253. 
