MOEPHOLOGT OF OPISTHOCOMUS CEISTATUS. 45 



brained forms, for instance the Parrot family, have opisthoccElous dorsals. Moreover, 

 classifying the Toothed Birds of Marsh by their sternum, the type that is one of the 

 Carinatse has a still lower kind of vertebral articulation than the opisthocoelous ; IcJitliy- 

 otviis has nearly all its presacrals amphicoelous ; whilst Hesperorms, which is one of 

 the Ratitse, has its presacrals cylindroidal. Nor is this all ; if Hesperof'ms has any 

 living descendants, or even representatives, these are the existing Loons (Colyrabidee) 

 and Grebes (Podicipedidse). 



But the Carinate Ichthyorms appears to me to be an ancient toothed Sea-Gull, one 

 of the Laridee ; and yet the existing Gulls are far more intelligent birds, birds of a 

 higher order than the Loons and the Grebes. Now the Gulls have not only opistho- 

 coelous dorsals, but the last presacral joint is nearly amphicoelous, as in their Cretaceous 

 quasi-ancestral relative the Ichthyornis. With regard to that character by which the 

 Struthious birds are most definitely marked off from the ordinary flying birds, namely 

 the flat breast-bone, correlated with almost useless wings, quite useless as organs of 

 flight, it is not too much to say that this is a modern and an acquired character. 



The fact that the vertebral chain in these low birds is longer than what is the average 

 in the Carinatse is a noticeable and important fact ; but they all fail in the caudal 

 region ; their free caudals are always fewer than in the Carinatge. Amongst the 

 Carinatse, however, there are some birds, namely the Swans, which have a longer 

 vertebral chain than the large Struthious birds. 



This abortive development of the tail is also manifestly an acquired character, like 

 the starving of the wings and sternal keel. But this is part of the same specialization of 

 these types in which all the strength goes to the hinder limbs, making them so perfectly 

 adapted for terrestrial life — in the case of the Ostrich especially, which is a bird as 

 exquisitely fitted for swift running as its desert companion the Wild Ass. 



As for what is most archaic and quasi-reptilian in the Ostrich tribe, that is to be 

 looked for in the skull and its contents. Those Carinatse that come nearest to these 

 birds, the Tinamous, have also an extremely small brain and low intelligence ; and in 

 some things, namely in the retention of cranial sutures and in the development of one 

 or even two rows of superorbital dermal bones, they are more reptilian than the Ratitse 

 themselves. But these birds, and the one treated of in this paper, Ojiisthocomus, have 

 retained good functional wings, and in the former, ihe Tinamous, a very large sternal 

 keel, although only the most Gallinaceous species has the sense to fly '. 



The Hoatzin {Opisthocomus) rather flits than flies ; its flying power is about equal to 

 that of the heavier Fowls (Turkey, Peacock, &c.) ; but it is an arboreal type, like its 

 nearest relatives the " Peristeropodous " Cracidte ; the Tinamous are the sub-struthious 

 relations of the Grouse, which are " Alectoropodous" — have a high hallux, and are 

 mainly terrestrial birds. 



It comes to this, namely, that birds, like men, must be classified by their brain power 

 ' See Frederic A. Lucas, Proc. U. S. Nat. ilus. 1886, pp. 157, 158. 



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