Fossil Secondary Mammalia, 21 



compare the capricious chance which has hitherto put us in 

 the exclusive possession of these seven jaws, with the equally 

 strange accident recorded by Dr Mantell, in his career of 

 discovery in the Wealden. He computed that in the course 

 of twenty years he had found teeth and bones of the Igua- 

 nodon which must have belonged to no less than 71 distinct 

 individuals, varying in age and magnitude from the reptile 

 just burst from the egg, to one of which the femur measured 

 24 inches in circumference. Yet it was not until the relics 

 of all these individuals were known that a solitary example 

 of part of a jaw-bone was obtained. As in other branches of 

 inquiry one invention or discovery usually elicits another of 

 the same kind, so, when at length the first Iguanodon's jaw 

 had been procured, a second was soon detected in a different 

 locality, and then the fragment of a third brought to light 

 from the stores of the British Museum. The solidity of these 

 jaw-bones, and the strength with which several teeth fixed 

 in them adhere to their sockets, render it more than ever in- 

 explicable why hundreds of similar detached teeth should 

 have been previously collected by naturalists without their 

 having fallen in with a single fragment of a jaw-bone. 



If it appear singular that the first terrestrial quadrupeds 

 of older date than the eocene strata should have been met 

 with in a marine limestone at the bottom of the oolite, rather 

 than in the freshwater strata of the "Wealden, where the re- 

 mains of herbivorous reptiles abound, I may observe that it 

 is not more strange than that no land shells should yet have 

 been discovered in the Wealden, or that no pulmoniferous 

 mollusca should have been met with, until the recent re- 

 searches of Messrs Bunker and E. Forbes had made them 

 known in Hanover and Dorsetshire. 



The last remaining point respecting the development of 

 the more highly organized vertebrata in the older rocks, on 

 which I propose to offer some comments, relates to the ab- 

 sence of Cetacea and of all marine mammalia in formations 

 more ancient than the eocene. I agree with Professor Owen, 

 that no argument founded on negative evidence, in favour of 

 the imperfect development of the class of vertebrata in the 

 earlier periods, is entitled to so much weight as the dearth 



