Geological Society. 101 



2. " On the Influence of the Advent of a Higher Forin of Life in 

 modifying the Structure of an Older and Lower Form." By Pro- 

 fessor Owen, C.B., F.E.S., E.G.S. 



In this paper the author, after referring to the general question 

 of the modification of the structure of organic forms produced by 

 the action of external influences, indicated that, in connexion with 

 this, changes in the nature of the prey of carnivorous animals ought 

 to be taken into consideration. He inferred that cold-blooded 

 aquatic animals formed a much greater propoition of the food of 

 Mesozoic than of Neozoic Crocodiles, and pointed out as connected 

 therewith the well-marked distinction between the amphicoelian and 

 proccelian type of vertebrae respectively characteristic of the two 

 groups. The proccelian character of the trunk-vertebrae better 

 adapts that part of the body to be sustained and moved in air, and 

 may be connected with the incoming in Tertiary times of mam- 

 malian prey inducing the Crocodiles to rush on shore. The Meso- 

 zoic Crocodiles were encased in a much stronger and more complete 

 dermal armour than their successors, doubtless for their protection 

 from the great Ichthyosaurs, Pliosaurs, &c., which coexisted with 

 them ; but as these passed away at the close of the Secondary 

 epoch, the armour of the proccelian Crocodiles has become more 

 scantj% and the diminution of weight and rigidity thus caused would 

 favour progression in air, and the rapidity of movement required for 

 capturing mammalian prey on land. The difference in the position 

 of the palato-nares, and in other related gular and palatal structures, 

 between the Mesozoic and Neozoic Crocodiles is apparently connected 

 with the power possessed by the latter of holding submerged a 

 powerful mammal without permitting the access of water to the 

 posterior nostrils and windpipe of the Crocodile ; and hence the 

 author is inclined to ascribe a fish-diet even to those massive-jawed 

 Crocodiles from the Purbeck (such as Goniopholis crassidens and 

 simus), which in some respects might seem fitted to grapple with 

 large and active mammals. The small size of the upper temporal 

 apertures in Tertiary and existing Crocodiles is regarded by the 

 author as a further proof in the same direction ; these apertures are 

 reduced by the progressive increase of the osseous roof of the 

 temporal vacuities, which again is correlated with increase in the 

 bulk and power of the temporal muscles, the main agents in biting 

 and holding. The differences in the length and strength of the 

 jaw, as a rule, testify in the same direction. Further, the fore limbs 

 in Mesozoic Crocodiles are shorter than in Neozoic species, indi- 

 cating that the former were more strictly aquatic in their habits, 

 the fore limbs in aU Crocodiles being closely applied to the body 

 during rapid swimming, and small limbs being less obstructive than 

 larger ones. On the other hand, they would be less efficient as a 

 means of progression on land ; and hence it may be inferred that the 

 advent in Tertiary times of mammals frequenting the water-side, 

 tempting the Crocodiles to make a rush upon the land to seize such 



