LIASSIC PTERODACTYLES. 501 



The crushed condition of many of tlie long bones in the specimens of Dimorpliodon 

 shows the walls of the shaft to have been compact and thin, the cavity large. Although I 

 have failed to detect such clear evidence of the foramen pneumaticum in these crushed 

 bones as in some of the vertebrae, I cannot resist the inference from the structure of the 

 long bones that they were filled with air in the living animal, as has been demonstrated 

 in remains of the larger Plerosauria of the Cretaceous sei'ies.^ 



This general osteological character of the Pterosauria leads me to offer a few ren)arks 

 on its relation to their peculiar power of locomotion among Beptilia, and to the affinity it 

 may indicate to other groups of volant Vertebrates. 



Weight is, of course, indispensable to directed motion through the air ; but, given 

 the weight requisite for the action against gravity resulting in flight, whatever structure 

 tends to dispense with additional burthen enables the force to act with more avail — with 

 less unnecessary resistance to overcome. 



Where provision is made for unusual flying force, as by the enormous pectoral muscles 

 and concomitant shape of wing in the Swift, the required weight of body called for heavier 

 bones ; hence the non-pneumaticity of the skeleton. Diminished flying force, especially 

 with increased bulk of body, is attended with modifications of bony structure obviously 

 adapted, and which have always been recognised in relation, to reduction of weight in the 

 mass to be moved through the air. It is true that the mere quantity of air contained in 

 bones would have an efiect inappreciable in aid of the force raising a weight of 5 lb. or 

 10 lb. from the ground ;" but the true view of the question is — given a bone of 1 foot in 

 length and 3 inches in circumference, whether the restriction of bony matter to a thin- 

 ness of ^ a line at the circumference, and a substitution of air for the rest of the diameter 

 throughout the shaft, be not a provision for diminution of weight and conservation of 

 strength which does relate to facilitate locomotion through air ? 



If the humerus of the Ostrich (No. 1373, Osteological Collection in the Museum 

 of the College of Surgeons, London, ' Catalogue' of do., 4to, 1853, p. 265) be compared, 

 as to weight, with the similarly sized humerus of the Argala Crane (No. 1107, ib., 

 ' Catal.,' p. 214), the difference is striking and suggestive; the latter bone being 

 "remarkable for its lightness, as compared with its bulk and seeming solidity" (ib., 

 ' Catal.,' ib.). I demonstrated the cause of the difference by a longitudinal section of 

 the two bones. In the Bird incapable of flight the humerus is solid ; in the Bird remark- 

 able for the long-continued power of soaring in upper regions of the air, the shaft of the 



1 Ante, p. 451, PI. xiii, fig. 2 p. 



' A writer impugning the physiological inference of Hunter and Camper, tlie discoverers of the 

 pneumaticity of the bird's skeleton, remarks : — "A living bird weighing 10 lb. weighs the same when dead, 

 plus a very few grains ; and all know what effect a few grains of heated air would have in raising a weight 

 of 10 lbs. from the ground. The quantity of air imprisoned is, to begin with, so infinitesimaily small, and 

 the diiference in weight which it experiences by increase of temperature so inappreciable, that it ought not 

 to be taken into account by any one endeavouring to solve the difficult and important problem of flight." — 

 Pettigrew, "On the Mechanism of Flight," ' Linnean Transactions,' vol. xxvi, p. 218, 1868. 



