210 Prof. R. Owen on the Occurrence 



occupy or line these depressions, they unquestionably do not 

 convey air into the osseous substance of the vertebra. 



In Omosaurus also there is a depression on each side of 

 the centrum, in the dorsal vertebrse, " beneath the base of the 

 neural arch " * ; but the osseous tissue is as in Cetwsaurus. 

 There are no cancelli to communicate with the lateral fossas. 



In the comparison of the vertebrse oi Poikilopleuron^ in which 

 the lateral fossse are wanting, with those of Cetwsaurus and 

 Omosaurus, it is noted that " ossification is incomplete and 

 large chondrosal vacuities are left in the substance of the 

 centrum, which, in the fossils, become filled with spar "f- 



It seemed reasonable therefore to conclude that in a verte- 

 bra combining the lateral fossae of Both'iospondylus with the 

 cancellous texture of Poikilopleuron the cancelli, filled with 

 spar in the fossils, might have been occupied by chondrine in 

 the living reptile. 



8. Belative Capacity of the Neural Canal. 



I could not, however, be satisfied with this conclusion or 

 opinion so long as there remained any test to which it might 

 be subjected. It may seem strange that the neural canal 

 should offer such test ; but I was attracted to this part of the 

 vertebra for the light it might throw on the point at issue. 



All existing air-breathing Vertebrates which have the bony 

 tissue of the centrum cancellous, especially so largely and 

 widely cancellous as in Chondrosteosaurus (with which, in 

 this character, birds of flight alone can be compared), and 

 which have such cancelli filled with air, are remarkable 

 for the frequency and vigour of their muscular actions ; and 

 such actions, in birds and bats, are correlated with powers of 

 flight. 



With this vital energy of the muscular system there is a 

 concomitant development of the nervous system, at least of 

 that division of the central chord which gives origin to the 

 motor stimuli ; and the size of the myelon affects that of the 

 neural canal. 



To this part, therefore, of the vertebrae of Chondrosteosaurus 

 my attention was directed, and, as related in my description J, 

 and shown in the figure §, that canal was singularly contracted 

 in proportion to the size of the vertebra (PI. X. fig. 2, n) . 



A similar narrow neural channel is figured in the view of 

 the anterior trunk-vertebra (copied from Prof. Cope's plate i. 



* Monogi'aph on Bothriospotidt/his, Pal. Soc. yoI. 187o, p. 48, pi. xii. 

 fig. 3,/. t Ibid. p. 28. 



X Mouogr. cited, Pal. Soc. vol. 1876, p. 6. 

 § Ibid. pi. iv. n. 



