ON BRITISH FOSSIL REPTILES. 201 



saurus ought to have given origin to the carnivorous mammalia, and the 

 herbivorous should have been derived from the Iguanodon, But where is the 

 trace of such mammalia in the strata immediately succeeding those in which 

 we lose sight of the relics of the great Dinosaurian Reptiles ? Or where, in- 

 deed, can any mammiferous animal be pointed out whose organization can 

 by any ingenuity or licence of conjecture, be derived Avithout violation of all 

 known anatomical and physiological principles, from transmutation or pro- 

 gressive development of the highest reptiles ? 



If something more than a slight inspection be bestowed upon the organic 

 relics deposited in the crust of the globe, we learn that the introduction of 

 mammalia on that crust is independent of the appearance of the highest forms 

 of Reptiles. The small insectivorous mammals of the lower oolite* are con- 

 temporary with the most ancient Dinosaur, and are anterior to the Iguanodon. 



The period when the class of Reptiles flourished under the widest modi- 

 fications, in the greatest number and of the highest grade of organization, is 

 past ; and, since the extinction of the Dinosaurian order, it has been declining. 

 The Reptilia are now in great part superseded by higher classes : Pterodac- 

 tyles have given way to Birds ; Megalosaurs and Iguanodons to cai-nivorous 

 and herbivorous mammalia ; but the sudden extinction of the one, and the 

 abrupt appearance of the other, are alike inexplicable on any known natural 

 causes or analogies. 



New species, genera, and families of Reptiles have constantly succeeded 

 each other, since the earliest periods in which the remains of this class can 

 be discerned ; but the change has been, upon the whole, from the complicated 

 to the simple. 



The Batrachian order, which is first indicated by the large and powerful 

 Crocodiloid Labyrinthodonts, has dwindled down to the diminutive and de- 

 fenceless Anourans and the fish-like Perennibranchians. The Saurian order 

 was anciently represented by Reptiles manifesting the Crocodilian grade of 

 organization under a rich variety of modifications and with great develop- 

 ment of bulk and power : it has now subsided into a swarm of small Lacer- 

 tians, headed by so few examples of the higher or loricate species, that it is 

 no marvel such relics of a once predominating group should have found a 

 humble place in Linnaus's Catalogue of Nature as coordinate members of 

 the genus Lacerta. 



Nevertheless some general analogies may be traced between the phaeno- 

 mena of the succession of Reptiles as a class, and those observed in the de- 

 velopment of an individual reptile from the ovum. Thus the Embryonic 

 structure of the vertebrae of the existing Crocodiles accords with the bicon- 

 cave type ; and this is exchanged, in the development of the individual as in 

 the succession of species, for the ball-and-socket structure as the latest con- 

 dition. 



The almost universal prevalence of the more or less biconcave structure of 

 the vertebrae of the earlier reptiles thus establishes a most interesting analogy 

 between them and the earlier stages of growth of existing reptiles. 



A similar analogy has been pointed out by M. Agassiz, between the hete- 

 rocercal fishes, which exclusively prevail in the oldest fossiliferous strata, 

 and the embryos of existing homucercal fishes, which seem to pass through 

 the heterocercal stage. 



The superior number of loricate Reptiles, and the more complete develop- 

 ment of the dermal armour in the Crocodilian genera Steneosaurus, Teleo- 



* For the proof of the often doubted mammalian character of the Thylacothcrium and 

 Phascnlotherium of the Stonestield slate, the reader is referred to the Memoirs in the Sixth 

 Volume of the Second Scries of the Geological Transactions, pp. 47-58. 



