ON BRITISH FOSSIL REPTILES. 203 



less dependent on the atmosphere, or oxygen, for existence. From their ex- 

 traordinary prevalence in the secondary periods, under varied modifications 

 of size and structure, severally adapting them to the performance of those 

 tasks in the economy of organic nature which are now assigned to the warm- 

 blooded and quick-breathing classes, the physiologist is led to conjecture that 

 the atmosphere had not undergone those changes, which the consolidation and 

 concentration of certain of its elements in subsequent additions to the earth's 

 crust may have occasioned, during the long lapse of ages during which the 

 extinction of so large a proportion of the Reptilian class took place. And if 

 the chemist, by wide and extended views of his science in relation to geology 

 and mineralogy, should demonstrate, as the botanist, from considerations of 

 the peculiar features of the extinct Flora has been led to suspect, that the at- 

 mosphere of this globe formerly contained more carbon and less oxygen than 

 at present, then the anatomist might, a priori, have concluded that the highest 

 classes of animals suited to the respiration of such a medium must have been 

 the cold-blooded fishes and reptiles. 



And besides the probability of such a condition of the zoological series 

 being connected with the chemical modifications of the air, the terrestrial 

 Reptiles, from the inferior energy of their muscular contractions, and still 

 more from the greater irritability of the fibres and power of continuing their 

 actions, would constitute the highest organized species, best adapted to exist 

 under greater atmospheric pressure than operates on the surface of the earth 

 at the present time. 



Through such a medium approaching in a corresponding degree to the 

 physical properties of water, a cold-blooded animal might even rise above the 

 surface and wing its heavy flight, since this would demand less energetic mus- 

 cular actions than are now requisite for such a kind of locomotion ; and thus 

 we may conceive why the atmosphere of our planet, during the earlier oolitic 

 periods, may have been traversed by creatures of no higher organization than 

 Saurians. If we may presume to conjecture that atmospheric pressure has 

 been diminished by a change in the composition as well as by a diminution 

 of the general mass of the air, the beautiful adaptation of the structure of 

 birds to a medium thus rendered both lighter and more invigorating, by the 

 abstraction of carbon and an increase of oxygen, must be appreciable by 

 every physiologist. And it is not without interest to observe, that the period 

 when such change would be thus indicated by the first appearance of birds 

 in the Wealden strata*, is likewise characterized by the prevalence of those 

 Dinosaurian Reptiles which in structure most nearly approach Mammalia, and 

 which, in all probability, from their correspondence with Crocodiles in the 

 anatomy of the thorax, enjoyed a circulation as complete as that of the Cro- 

 codile when breathing freely on dry landf. 



* Foot-prints alone, like those termed ' Ornithichnites,' observed in the New Red Sand- 

 stone of Connecticut, are insufficient to support the inference of the possession of the 

 highly developed organization of a bird of flight by the creatures which have left them. The 

 Uhynchosaur and biped Pterodactyles already warn us how closely the ornithic type may be 

 approached without the essential chai-acters of the Saurian being lost. By the Chirotherian 

 Ichnolites we learn how closely an animal, in all probability a Batrachian, may resemble a 

 pedinianous mammal in the form of its foot-prints. 



The degree in which flying insects can resist noxious gases, which would be quickly fatal to 

 the wai-ni-blooded Verteljrates, invalidates the objection to a pi'ogressive change of atmosphere 

 having accompanied the prevalence of quick-breathing animals, which might be suggested by 

 the Libellutee of the has and by the oolitic Beetles. 



t All existing Reptiles, which have the ribs at the anterior part of the thorax united by 

 a head and tubercle to the centrum and neurapophysis of the vertel)rpe, have a heart with 

 two distinct ventricles as well as two auricles. The contiguous aortic aiising from the two 

 ventricles intercommunicate by an aperture so placed as to be covered by the sigmoid valves 



