Page:Quarterly Journal of the Geological Society of London, vol. 34.djvu/508

426 426 PE0F. R. OWEN ON THE MODIFYING INFLUENCE

conclude with reference to a limb-character distinguishing the Arn- phicoelia from the Promlia.

In all the Mesozoic Crocodilia of which the skeleton has been sufficiently restored, the fore limbs are shorter in proportion to the hind limbs than they are in the similarly restored Neozoic species *. The difference relates to the more strictly or uniformly aquatic life of the Teleosauroids. This I deduced from the fact that when a Miotic Crocodile swiftly swims to catch a prey or escape a danger the fore limbs are closely applied to the trunk. The same motion- less and unobstructive disposition of the fore limbs has been ob- served in the marine lizards of the genus Amblyrliynchus-f.

But the resistance to rapid swimming from fore limbs when so disposed will be the less as the size of such limbs may be dimi- nished.

Thus the Teleosauroids, in their rush after fishes or retreat from Ichthyosaurs, would be favoured by the character of the fore limbs above adduced.

On the other hand their progress on dry land would be more diffi- cult, unless, like the Dinosaurs with similarly stunted fore limbs, they were able, as has been surmised, to run upright on their hind legs, as shown in a photograph of the " Bestorations of North-American Dinosaurs and Bythonomorphs " exhibited in the " Central Bark of New York." I am, however, disposed to see in the Teleosaurian proportions of the fore limbs in the long-natatory- tailed Iguanodon and Hadrosaur a condition facilitating their locomo- tion in water.

But, returning to my immediate argument. To what condition, it may be asked, do the augmented size and strength of the fore limbs in Neozoic Crocodiles relate ?

The advent in Tertiary time of large mammalian quadrupeds brows- ing or prowling along the shores of estuaries and banks of rivers haunted by such Crocodiles might, and does, tempt them to make a rush on the dry land to seize such passing prey. In these rushes the fore limbs come into strenuous play as terrestrial locomotive organs.

A Lamarckian might say that the temptation to such locomotipn, by the repeated increased exertion and exercise of the fore limbs, would lead in the course of generations to their augmentation of size; and he would set it down as one of the factors in the transmuta- tion of a Teleosaur into an Alligator. His opponent might call for the evidences of the transitional forms.

It is true that, in regard to the general shape of the head, some of the later Mesozoic Crocodilia approach, exceptionally, the more robust proportions which prevail, as a rule, in Neozoic Crocodilia, and that among these latter there are species which exceptionally show the more slender proportions which prevail, as a rule, in the Mesozoic

of Teleosaurus and Gavialis).
 * < History of British Fossil Eeptiles,' Crocodilia, pi. 1, 4to, 1850 (skeletons

t Darwin, ' Voyage of the ' Beagle ;' ' Journal of Researches ' &c, 12mo, 1845, p. 386.