of the Skull and the Skeleton. 351 
influence of the muscles, it is obvious that those muscles which 
are inserted at the extremities of bones must exercise a powerful 
influence on the formation of epiphyses. Therefore epiphyses 
and processes are to be looked for wherever the pressure and 
tension on a bone become more than sufficient to continue ossi- 
fication. Now just as ossified epiphyses are not to be found in 
bones where the pressure at the ends is small, so it would be 
expected that in cases where the pressure and tension of the 
bone is almost entirely at the ends, and the shaft does not 
support the animal, the epiphyses should be enormously large, 
while the shaft would be smal]. And in Plesiosaurs this is 
actually found to be the case; for the large limbs, swimming 
powerfully through the yielding water, have experienced an 
enormous lateral tension at the ends of the long bones without 
any greater pressure in the direction of length. And therefore 
it happens that the ends of the epiphyses which are attached to 
the shaft become conical and penetrate down the girdling shaft 
till they meet im the middle of the bone; and, as might be anti- 
cipated, that of the distal end is much the larger one. There- 
fore it would seem possible, if the muscles attached were small, 
and the bones so placed as only to experience tension and no 
direct pressure, that the shaft might altogether disappear, and 
only the two epiphyses remain, as I am inclined to suggest 
may be the case with the bones which are called tarsal and 
carpal—a conclusion to which I am led by a consideration of 
the bones called the tarso-metatarsus in birds, which may be a 
case in which the tarsus does develope a shaft; and if so, then 
the metatarsals, like the phalanges, as is usual in the other 
Sauropsida, will be applied to the ground. There can be no 
@ priori reason for supposing that the tarsals and metatarsals 
should unite together to form one bone; and all the facts of 
osteology point ‘to their remaining separate ; while an erect po- 
sition for the metatarsal bone in a clawed animal is unusual, 
and only partial even in jumping jerboas, which it characterizes. 
The careful dissections of the leg in the ostrich and crocodile 
&e. by Dr. S. Haughton enable me to add a little evidence from 
the: muscles. The gastrocnemius muscle in the crocodile, as is 
usual, is inserted in the os calcis (and tarsal bones). It weighed 
0:14 oz., while the tibialis anticus and extensor digitorum com- 
munis weighed 0°11 oz. But in the ostrich the gastrocnemido- 
soleus is inserted into the middle of the so-called tarso-meta- 
tarsal bone, and weighs 1154 oz., while all the other muscles of 
the limb and those attached from it to the body only weigh 
220 oz., the tibialis anticus and extensor digitorum communis 
weighing 14 0z. Now there is nothing to induce us to expect 
that the gastrocnemius would be inserted in the metatarsal bone, 
