342 Prof. H. G. Sceley on a Pneumatic Type 



known osteological result in excavation of a bone in a bird, 

 what is there in the vertebra of a Dinosaur to suggest that 

 similar effects have been produced by dissimilar causes? 

 And it would be interesting to find in an extinct order of 

 animals evidence that an agency unconnected with the lungs 

 produced results which differ from those in birds only in 

 being the effect of larger areis of pressure acting laterally 

 upon the sides of the vertebrae. But the evidence that there 

 was any essential difference in the origin of these structures 

 in Dinosaurs and birds is not forthcoming ; and if it ever 

 existed is lost with the soft parts of the animal. 



Nevertheless cavities are formed in certain bones in animals 

 of varied organization, which are not connected with the lungs 

 in the manner of air-cells of birds, but they are chiefly in 

 the skull. They are slightly developed in existing reptiles, 

 but are most conspicuous in warm-blooded animals. The 

 skulls of elephants exhibit a maximum development of 

 pneumatic cavities which have no connexion with the lungs, 

 and the texture of these bones closely approximates to that 

 of cellular vertebrae in some Cetiosaurian Dinosaurs, such as 

 Ornithopsis and its American representatives. The resem- 

 blance between the mammal skull and the reptile vertebra is 

 one of analogy. There are no facts to support the inference 

 that the cause which expanded the cranial bones of the 

 elephant and other mammals is identical with that which 

 absorbed and excavated the bony tissue, but did not augment 

 the size of the cervical and dorsal vertebrae of Dinosaurs. 

 There is no basis for comparison between the conditions in 

 mammals and these extinct reptiles, for no mammal shows a 

 pneumatic vertebral column which can be compared with 

 these Dinosaurs; and when a mammalian vertebra is hollow 

 it is not comparable, since there is no pneumatic foramen. 



On the other hand, Dinosaurs are not conspicuous for 

 pneumatic cavities in bones of the skull, and there are there- 

 fore no facts to suggest the idea that they might by analogy 

 develop a pneumatic vertebral column which was not con- 

 nected with the lungs, even if the cranial and vertebral 

 pneumatic structures had been comparable. 



The influence of the lungs as a whole in modifying the 

 vertebral column of a reptile is manifest in the dorsal 

 vertebi'ge of Testudinata. In tortoises, under conditions of 

 terrestrial life, the lungs have expanded and given the 

 carapace a remarkable elevation. At the same time the 

 neural arches have become raised, and the lungs have 

 pressed evenly against the sides of the centra of the vertebras 

 till they have become narrowed into thin plates by the tissue 



