204 ^ REPORT— 1841. 



The first 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 respiration than the rest of the class ; and the earliest discovered 

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

 actually the jaws of such species, with which are combined the characters 

 of that order, Marsupialia, which is most nearly related to the oviparous Ver- 

 tebrata. 



The present speculations are, however, offered with all due diffidence ; the 

 collection of the evidence requisite for pursuing 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 considerations of the 

 nature and possibilities of such atmospheric changes as may be physiologi- 

 cally connected with the variations of organic nature made known by the re- 

 searches of the anatomist. 



A too cautious observer would, perhaps, have shrunk from such specula- 

 tions, although legitimately suggesting themselves from the necessary rela- 

 tions between the organs and media of respiration ; but the sincere and ardent 

 searcher after truth, in exploring the dark regions of the past, must feel him- 

 self bound to speak of whatever a ray from the intellectual torch may reach, 

 even though the features of the object should be but dimly revealed. 



when blood is transmitted equally through them. When the amphibious Crocodile suffers 

 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 over- 

 charged 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 transmitted through the left aorta to the head 

 and anterior extremities. The Dinosaurs, having the same thoracic structure as the Croco- 

 diles, 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 characterizes the warm- 

 blooded Vertebrata. 



ERRATA. 



Page 64, 14 lines from bottom, ybr ' Coelospondylian' read ' Amphicoelian.' 



67, 25 lines from top, ./or ' bifurcate' read ' biporcate.' 



88, after the 3rd line from top, insert ' with convexo-concave vertebrae.' 



104, 14 lines from bottom,/or ' Cloeospondylian ' read ' Amphicoehan. 



