LIASSIC FORMATIONS. 
79 
The crushed condition of most of the long bones in the specimens of Bimorphodon 
show the wall 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 Pterosaitria of the Cretaceous series.* 
This general osteological character of the Bterosauria leads me to offer a few remarks 
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 effect inappreciable in aid of the force raising a weight of 5 lb. or 
1 0 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 5 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 
1 ‘Monograph on the Fossil Reptilia of the Cretaceous Formations (Order Pterosauria),' (‘Pal. Soc. 
Mon.’, vol. v), 4to, 1851, pp. 80, 98, 101. 
2 A writer impugning the physiological inferences of Hunter and Camper, the 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 gi’ains ; 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 infinitesimally small, and 
the difference 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. 
