THE FAMILIES AND GENERA OF BATS. 
19 
muscles and strengthening the arch by which the body is suspended 
to the wings during flight. Somewhat remarkably, however, the 
largest species, the great Pteropines,® in which the mechanical diffi- 
culties would be expected to be greatest, show little specialization 
except for the rather unusual development of the keel, particularly 
on mesosternum, a peculiarity which is most pronounced in the larger 
members of the group. The elements of the first rib and its attach- 
ments remain distinct, and except for the usual Chiropterine pecu- 
liarities show no characters worthy of note. Essentially the same 
conditions, though with much less development of the keel, are 
found in most of the Microchiroptera. So far as my observations 
go it is constant in the Rliinopomidse, Emballonuridse, Roctilionidse, 
Phyllostomidse, Desmodontidse, Myzopodidse, and in most of the 
Vespertilionidse. The first stage in the strengthening process is 
found in the Molossidse, the Mystacopidae and in one subfamily of 
Vespertilionidse, the Tomopeatinse. It consists in the slightly greater 
thickening of the first rib, and the fusing of the last cervical and 
first dorsal vertebrae into a solid ring. There is no appreciable 
change in the form of any of the bones, and the keel of the mesoster- 
num remains very slightly indicated. A peculiar modification of this 
process occurs in the Thyropteridae where the fusion takes place 
between the first and second dorsals, the last cervical remaining free. 
In the Natalidae it is again the last cervical and first dorsal that fuse; 
but in this family the supporting arch is further strengthened by a 
noticeable widening of the horizontal lobes of the presternum, so 
that the greatest anterior width exceeds the length' of presternum and 
mesosternum together. The first rib is also thickened and its sternal 
segment is greatly reduced in length. Keel of mesosternum relatively 
as high as in the Pteropodidse, and a slight keel on xiphisternum. 
The most remarkable series of changes occurs in the four related 
families Nycteridse, Megadermidse, Rhinolophidse, and Hipposide- 
ridse. In the Nycteridse the conditions are not very different from 
those in the Ratalidse. The keel is equally high and it extends simi- 
larly on the short, wide xiphisternum. The presternum is, however, 
less broadened and strengthened, and the last cervical vertebra is not 
fused with the first dorsal. In the Megadermidse the mesosternum 
and xiphisternum are of the ordinary type, but the presternum is 
greatly widened, so that it is essentially a heavy transverse bar with 
a slight median backward projection. Laterally it is fused with the 
first rib, so that the only break in the continuity of the ring is at the 
point of articulation of the ribs with the vertebrae. As in the Kycte- 
«The weight of an alcoholic specimen of Pteropus from Tenasserim (Cat. No. 
104451, U. S. N. M.) is 739.5 grams, while that of a Rhinolophus ferrumequinum 
from Genoa, Italy (Cat. No. 18472, U. S. N. M.), is only 16.8 grams. Yet in the 
smaller animal the strengthening process is carried to the extreme. 
