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 



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

 of Teleosaurus and Gavialis). 



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

 p. 386. 



