81 
for the outer and middle toes is also relatively narrower and deeper ; the posterior com- 
mencement of the middle condyle projects further and more abruptly in m3 than in 
m 1; the posterior part of the distal half of the bone is more convex. 
These may perhaps be deemed by some Ornithologists to be slight or trivial differ- 
ences ; yet, taken in connection with the greater breadth and thickness of the bone, in 
proportion to its length, they unquestionably support the conclusions of specific distinc- 
tion deducible from those proportions. 
The Physiologist contending for a difference of age merely in the birds to which the 
bones m 1 and m3 belonged, must be prepared to show that in other large Struthious 
birds the tarso-metatarsal bones alter in their proportions as well as their size in the 
progress of growth, and that they are thicker and more robust in the young than in the 
old birds. The contrary, however, is the case in the Ostrich and the Common Fowl. 
In the great existing Struthious bird more especially, which offers the most instructive 
analogy in the present comparison, the tarso-metatarsal bone is relatively more slender 
in proportion to its length in the young bird than in the old, at least at the period of growth 
when the tarso-metatarsal bone has attained two-thirds its full size, which is precisely 
the proportion which the bone of the Dinornis m3 bears in length to the bone m 1, 
But the comparison with the bones of the young Ostrich brings to light another cha- 
racter, which effectually decides the question of the relation between the two different- 
sized bones of the Dinornis under consideration. In all birds the tarso-metatarsal bone, 
as is well known, is an aggregate of several distinct ossicles, the primitive separation of 
which continues longest in those birds whose respiratory, circulating and muscular 
energies are least developed. Thus in the Penguins the three metatarsal bones are al- 
most quite distinct from one another throughout life; and in the Ostrich and other 
Struthionide deprived of the power of flight, the primitive separation of the metatarsals 
continues at their extremities to nearly full growth. In the tarso-metatarsal bone of the 
young Ostrich', which is figured to illustrate this condition, and which is rather more 
than two-thirds the length of the same bone in the mature bird, the tarsal bone, which 
seems to represent a proximal epiphysis, is still a detached bone, and the posterior 
channel of the metatarsus deepens and widens as it approaches the proximal extremity, 
and is finally lost in the two deep and narrow clefts which divide the proximal ends of 
the three constituent metatarsals from each other. 
But the tarso-metatarsal of the Dinornis, m 3’, has all the characters of the bone of not 
only a mature but an aged bird, The tarsal bone is completely confluent with the 
upper ends of the metatarsals, and these are blended with each other, as far as their 
diverging distal condyles. The traces of the proximal separation are limited to a rough 
depression and a round excavation above it, on the anterior part of the bone, and to the 
two small perforations on the posterior part, the relics of the original fissures. He, 
therefore, who would contend that the tarso-metatarsal bone m 3 has belonged to a young 
' Pl, XXVIII, fig. 1 & 2. 2 Pl. XXVII. fig, 2, 
