206 



PROF. H~. a. SEELEY OX A SACRUM OF A 



17. On a Sacrum, apparently indicating a new type of Bird, Orni- 

 thodesmus cluxiculus, Seeley, from the Wealdex of Brook. 

 By H. G. Seeley, F.R.S., F.G.S., Professor of Geography 

 in King's College, London. (Read March 9, 1887.) 



[Plate XII.] 



The discovery of the pneumatic coDdition 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 vertebras 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 vertebras 

 found in the Archceopteryx 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 osteological 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 vertebras 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 vertebras which were 

 originally dorso-lumbar, and the five posterior vertebras which were 

 originally caudal. When the number of vertebras 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 

 vertebras, 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 osteological 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 vertebras of the sacrum (PI. XII. figs. 2, 3), 

 the typically Avian saddle-shaped intervertebral articulation (fig. 4), 

 and the large number of vertebras commonly found in a sacrum in 

 existing birds. It is in the Pox 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 than the specimen now shows. 



