PROFESSOR OWEN ON THE GENUS DINORNIS. 243 



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 m 3 than in 

 Jnl ; 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 is however 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 

 m 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 m 3 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 respiratorv, circulating and muscular 

 energies are least developed. Thus in the Penguins the three metatarsal hones are al- 

 most quite distinct from one another throughout life ; and in the Ostrich and other 

 Struthionidcu 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 (PI. XXVIII. fig. 1 &2.), 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 hmited 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 



' PI. XXVII. fig. 2. 

 VOL. III. PART III. 2 K 



