WEALDEN CROCODILES. 639 



fast, as in Mammalian Carnivora, and certain teeth of the dental series have accordingly- 

 received the name of canines in the Crocodiles with such analogous dentition. Partial 

 developments of the alveolar borders concomitant with the modified dentition give a 

 festooned course or contour to those borders. 



In Mesozoic Crocodiles this character is exceptional and begins to appear at the 

 Wealden period. The oolitic and older Amphicoelians have more numerous, smaller, 

 and sharper teeth, occupying straight, or nearly straight, alveolar borders of the jaws. 

 One genus, Gavialis, still exists, which exceptionally exemplifies the old dental fish- 

 catching character. 



Finally, in reference to the limb-character as distinguishing the AmphicwUa from 

 the Proccelia. This character, at least, is exemplified in all the Mesozoic Crocodilia, of 

 which the skeleton of the same individual has been sufficiently restored. It is then 

 manifested by the shorter and smaller proportions of the fore pair of limbs as compared 

 with the hind pair than we find in existing Crocodiles, and in the similarly restored 

 skeletons of extinct Neozoic species {Crocodilia, PI. 11). 



The diff'erence in question I take to relate to the more strictly or uniformly aquatic 

 life of the Teleosauroids. 



When the nilotic Crocodile darts under water after a prey, or swims off swiftly to 

 escape a danger, the fore-limbs take no part in the action, but are closely applied prone 

 to the trunk. The same motionless and unobstructive disposition of the fure-limbs has 

 been observed in the still-surviving marine lizards of the genus Amhlp-hpichus. 



But the resistance to rapid swimming of fore-limbs so disposed is calculable accord- 

 ing to the degree in which they break the uniformity of the curve and project beyond the 

 surface of the fore part of the body to which they may be applied. The smaller, there- 

 fore, such limbs may be and the less will they obstruct the forward course of the 

 Crocodile. 



Thus, the Mesozoic Amphicoelians in their rush after fishes, or retreat from attacking 

 larger Reptilia, would be favoured by their limb-character. On the other hand, their 

 progress on dry land would be more difiicult, unless, as has been suggested in regard 

 to some kinds of Dinosaur with similarly stunted fore-limbs, the Teleosaurs were able to 

 run upright on their hind-legs. 



But dismissing such interpretation of the dwarfed fore-limbs of Mesozoic Crocodilia, 

 to what conditions, it may be asked, do the augmented size and strength of the§e limbs 

 in Neozoic Crocodiles relate ? 



The advent in tertiary times of large Mammalian quadrupeds browsing or prowling 

 along the banks of estuaries and rivers haunted by such Crocodiles might, and does, tempt 

 them to make a rush on the dry land to seize such passing prey. In such rushes the 

 fore-limbs come into strenuous action. 



A Lamarckian might say that this temptation to terrestrial locomotion would, by the 

 repeated increased exertion and exercise of the fore-limbs lead, in the course of genera- 



196 



