216 COPE: [Vot. III. 
difficulty ; the Manidz are very similar in this respect to the 
Anteaters. The same is true of the African Orycteropus. In 
spite of this embarrassing circumstance, I cannot but think that 
the explanation of the xenarthrous structure above offered is the 
correct one, and we only require more knowledge of the phylog- 
eny of the Old World Nomarthra, and of the habits of all the 
forms, to learn the cause of this discrepancy. One of several 
possible explanations may turn out to account for it. Such 
might be the recent adoption of the habit of rolling on the part 
of the Manidz; or the feebleness of the species of that family 
forbidding their growth energies to respond to the stimulus. Or, 
on the other hand, the existence of some habit of vertebral flex- 
ure in the ancestors of the existing Xenarthri, which is unknown 
to us, may be the cause. 
4. THE SCAPULAR ARCH. 
® 
There is aclose relation between the condition of the Mam.- 
malian clavicle and the use of the anterior limb. As a primitive 
element of the vertebrate skeleton, Mammalia naturally inherit 
it from their reptilian ancestors. Its absence in many of the 
former may be attributed to disuse. The use of the clavicle is 
to strengthen the scapular arch in the transverse direction ; that 
is, against lateral strains both of pulling and pushing, which 
come almost entirely from the use of the anterior limbs. In 
Mammalia, where the use of the limbs, and consequently the 
direction of strain, is vertical, the clavicles are wanting, as in all 
Ungulata except Quadrumana. (The condition of the Condylar- 
thra as to clavicle is unknown.) In digging mammals the clavi- 
cles are generally developed directly as the fossorial power, and 
especially as the manner of digging is horizontal or vertical. In 
the mole the digging is horizontal, and here the clavicle is ex- 
cessively wide and short. In the armadillos, where the move- 
ment of the fore legs is vertical, the clavicles are both long and 
stout. In Carnivora the clavicle is rudimental, but where pres- 
ent the species are vertical diggers. In types of aquatic habit 
clavicles are wanting. Such are the insectivorous genus Mytho- 
mys, the otters, seals, and the Cetacea. These animals all have 
more or less lateral movement of the fore legs, but the resist- 
ance of water, the medium in which they move, is greatly less 
