1885.] The “Collar Bone” in the Mammalia. 1173 
to the amount and character of the work done. The clavicle 
first appears in the ganoid fishes as a secondary apparatus devel- 
oped in connection with the primary cartilaginous scapular arch. 
In the teleost or bony fishes it becomes a more important element 
in the shoulder girdle, having connection with the skull and the 
opposite sides joining in the ventral median line. 
It undergoes a reduction and becomes of less significance in 
Amphibia and Reptilia, but in birds it assumes an important 
position in relation to flight, the entire shoulder girdle in fact 
being specialized to meet the conditions incident to aérial loco- 
motion. The coracoids—large, strong bones—act as braces ; the 
clavicles, peculiarly modified, are united at their sternal ends into 
one bone, the furculum, or “ merry thought,” which, as Owen has 
pointed out, acts as an elastic, bony arch opposing the force inci- 
dent upon the downward stroke of the wing in flight, thus aiding 
the humeri, or arm bones, to regain their former position in the 
succeeding upward or counter stroke with as little loss of time 
and energy as possible. In terrestrial forms (grouse, fowls, etc.), 
where flight is sustained for short intervals only, the arch is nar- 
rower and the structure more slender and delicate. 
Taking up the Mammalia with a view to ascertaining the rela- 
tionship existing between the development of the clavicles and 
the work done by the fore limbs, let us start with the following 
general proposition as a basis for our observations, namely, that 
those animals which have the fore limbs specialized over the hind 
limbs in relation to work, possess a clavicle, and where the hind 
limbs are the most highly specialized, the clavicles are rudiment- 
ary or entirely wanting. 
Leaving the ornithodelph mammals (the Australian duck-bill, © 
Echidna, etc.), with their bird-like shoulder girdle, we note the 
presence of more or less well developed clavicles in all the mar- 
supials, with the single exception of the “ bandicoots” (Pera- 
melidz), where it is wanting, and its absence may be accounted 
for from the fact that the bandicoots from their terrestrial mode 
of life, nesting in hollow places and feeding on insects, roots 
etc., have for along time had their fore limbs subjected to less 
complex conditions than the arboreal and predatory families, the 
opossums (Didelphide), the Dasyuride, the phalangers (Phalang- 
istidæ) and the kangaroos (Macropodidz), which constitute the 
rest of the order. In the kangaroos the clavicles, though pres- 
