b 



Professor Owen on British Fos^l ^^ptiles, S7 



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 croco- 

 dile when breathing freely on dry land.* 



The lirst indications of the warm-blooded classes, it might 

 be anticipated, would appear, if introduced into the reptilian 

 era, under the form of such small insectivorous mammals, as 

 are known at the present day to have a lower amount of re- 

 spiration than the rest of the class ; and the earliest discovered 

 remains of mammalia, as for example, those in the Stonesfield 

 oolite, ai*e actually the jaws of such species, with which are 

 combined the characters of that order, Marsupialia, which is 

 most nearly related to the oviparous vertebrata. 



The present speculations are, however, offered with all due 

 diffidence ; the collection of the evidence requisite for pursu- 

 ing them to a semblance even of demonstration is only just 

 begun, and they are thrown out with no other expectation of 

 utility than as incentives to the chemical consideration of the 

 nature and possibilities of such atmospheric changes as may 

 be physiologically connected with the variations of organic na- 

 ture made known by the researches of the anatomist. A too 

 cautious observer would, perhaps, have shrunk from such spe- 

 culations, although legitimately suggesting themselves from the 

 necessary relations between the organs and media of respira- 

 — _ — — . ..^ . „— -. 



* All existing reptiles, which have the ribs at the anterior part of the 

 thorax united by a head and tubercle to tlie centrum and neurapophysis of 

 the vertebra?, have a heart with two distinct ventricles, as well as two auri- 

 cles. The contiguous aorta) arising from the two ventricles intercommunicate 

 by an aperture so placed as to be covered by the sigmoid valves when blood 

 is transmitted equally through them. When the amphibious crocodile suf- 

 fers an interruption in the pulmonary circulation by continued submersion, 

 the aorta from the left ventricle, by the communication above mentioned, 

 receives venous blood from the overcharged cavities of the right side of the 

 heart; but when respiration is in full vigour on dry land, an undiluted 

 stream of arterial blood is tnuismitted through the left aorta to the head 

 and anterior extremities. The Dinosaurs, having the same thoracic struc- 

 ture as the crocodiles, may be concluded to have possessed a four-chambered 

 heart ; and, from their superior adaptation to terrestrial life, to have enjoyed 

 the function of such a highly organized centre of circulation, in a degree 

 more nearly approaching that which now characterise* the warm-blooded 

 vertebrata. 



