Licence: Public Domain Mark
Credit: On a sacrum of a bird from the Wealden of Brook. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![On a Sacrum, apparently indicating a new type of Bird, Orni- thodesmus cluniculus, Seeley, from the Wealden of Brook. By H. G. Seeley, F.R.S., F.G.S., Professor of Geography in King’s College, London. [Plate XII.] The discovery of the pneumatic condition of the vertebrae in Orni- thosaurs and certain Dinosaurs showed that they diverge from Reptiles in structural characters which are typically Ornithic. The augmented number of vertebral in the sacrum in both those groups also shows a divergence from existing Reptiles, which is markedly Avian. On the other hand, the small number of sacral vertebrae found in the Archaeopteryx has proved that a bird may have the sacrum no more complex than in Ornithosaurs and Dinosaurs. The bird’s sacrum formerly had a simpler structure, just as the intervertebral articulation was simpler; and we expect to find some of the distinctive ostcological attributes of existing birds wanting among earlier representatives of the class. This would also be a fair inference from Prof. Huxley’s exposition of the sacrum of the fowl. Sacral veitebroc are defined by their nerves uniting to form the sacral plexus; judged by this test, the fowl has five sacral vertebrae. But ossification has extended beyond them, so as to incorporate in the sacrum the four anterior vertebral which were originally dorso-lumbar, and the five posterior vertebras which were originally caudal. When the number of vertebrae is reduced, the structure of the sacrum may be simplified. At the present day the most striking character of a bird’s sacrum is the absence of trans- verse processes extending outward from the bases of the true sacral vertebrae, so that deep depressions are formed in its middle region, comparable to the entire sacrum in some Ornithosaurs. These depressions contain the middle lobes of the kidneys, and therefore may be presumed to be due to the way in which the development of those organs governed the ossification. Hence the presence or absence of this ostcological character would imply no more than a slight difference in the deep-seated condition of the kidney ; so that the renal recess might be wanting, without implying any important difference in organization. The specimen which I am about to describe wants the modified renal recesses of the sacral vertebra of the sacrum (PI. XII. figs. 2,3), the typically Avian saddle-shaped intervertebral articulation (fig. 4), and the large number of vertebrae commonly found in a sacrum in existing birds. It is in the Fox collection of the British Museum, and is distinguished by the number This sacrum is 9-6 centimetres long and slightly curved, so as to be concave in length on the ventral aspect (fig. 3), though the original curvature may have been less thau the specimen now shows.](https://iiif.wellcomecollection.org/image/b22412554_0006.jp2/full/800%2C/0/default.jpg)
