1174 The “Collar Bone” in the Mammalia. (December, 
ent, are slender and delicate, the weak fore limbs coming into use 
in manipulating the “pouch,” etc., thus throwing a variety of 
motion into the shoulder joint, which explains the presence of 
clavicles in an animal whose locomotion is almost entirely per- 
formed by the hind limbs. 
Among the Insectivora, a large order of wide geographical 
range and rather uniform life, the majority of species being ter- 
restrial and fossorial or burrowing, we have the clavicles well 
developed with but one exception, that of Potamogale velox, a rare 
form from Western Africa, and this is the only insectivore which 
is almost entirely aquatic. It measures about two feet in length 
with a long cylindrical body, tapering continuously into a thin, 
laterally compressed tail, which is the main propulsive power 
when swimming, the short legs with their unwebbed feet drifting 
back against the body. Another form, Myogale,.from the streams 
_of Southeastern Russia, is natatorial and possesses a clavicle, but 
the feet are all webbed and come into play along with the tail as 
organs of propulsion. 
The moles, Talpidæ, are eminently fossorial, their excavations 
being everywhere known; an East Indian squirrel-like form is 
arboreal ; all the rest are terrestrial and fossorial. 
The phe Edentata is divided into two primary groups, the 
“leaf-eaters”” (Phytophaga) and the “ insect-eaters ” (Entomoph- 
aga). The P Phytophaga comprise the “ sloths,” curious, arboreal 
forms inhabiting the South American forests, represented by only 
two living genera ; they are entirely arboreal, making character- 
istic progression among the tree-tops on the leaves of which they 
feed. In the two-toed sloth, or “ nnau ” (Cholcepus), the clavi- 
cles are well developed ; in the “ai,” or three-toed variety (Brady- 
pus), they are small, rudimentary, having lost their sternal attach- 
ments and evidently undergoing a retrogressive change, due 
probably to some variation in the animalľ’s life and habits which 
brings the part into less active use. 
In the Entomophaga (ant-eaters, armadillos, a we find the 
7 Fiaeicies fully developed in the climbing two-toed ant-eater Cy- 
hurus, a small South American species; also in the Cape ant- 
a ester, or “aard-vark,” Orycteropus, a burrowing form from the 
ae of Good Hope; while in the great ant-eater, Myrme- 
phaga, a strictly terrestrial form but not fossorial, the clavicles 
t, m ae ens open the ant hills and termites’ 
