342 PKOE. OWEN ON THE SKULL OE MEGALOSATJKUS. 



brates is the heat of the body maintained at so high a degree as in 

 that of Birds : in Mammals it rarely exceeds, and not always reaches 

 100° Fahr. ; in Birds it ranges, according to species, from 106° 

 (Ajpteryx) to 112° (Eagle). 



This thermogenous character involves a tegumentary one. The 

 body-warmth of a Mammal is protected, as a rule, to which bats are 

 no exception, by a covering of wool or hair, or both, the body- 

 warmth of a bird by a clothing of down and feathers ; and it is the 

 latter that are pressed into the mechanism of the organs of flight ; and 

 their presence is demonstrated by impressions in a suitable matrix of 

 the secondary extinct species of the class *. 



If he had never had a recent bird to dissect, the physiologist would 

 have inferred, and rightly, that the heart had partaken, in its 

 structure, of all the perfections which characterize that of the 

 warm-blooded mammalian class ; and he knows, ex visu, that it 

 superadded structures, in the muscularity of certain of the heart- 

 valves, which relate to the higher activities of the bird. Con- 

 comitantly do the organs of respiration manifest their highest grade 

 of development. Not only are the lungs, as in Mammals, " spongy," 

 i. e. subdivided into multitudinous minute cells, multiplying the 

 surface on which the pulmonary capillaries are exposed to the 

 oxygenating atmosphere, but other cells are continued from the 

 lungs proper, and submit, in different degrees, the systemic capillaries 

 to the action of the air inhaled. So birds are said to enjoy a double 

 respiration as well as circulation. 



In no reptile is the arterial blood, as it is returned from their 

 large-celled lungs, distributed pure and unmixed with venous blood 

 to the general system f ; in none is the function of respiration so 

 complete, so extensive, as in the warm-blooded classes. Impressions 

 of the sheets of skin have been preserved, which the long and 

 slender finger-bones of the Pterodactyle were destined to support ; 

 but not a trace of non-conducting hair or protecting feather has 

 been detected therewith in any, the most conservative, beds. 



I consequently infer a correlated inferiority of structure in the 

 circulating and respiratory systems, and that the Pterodactyles were 

 as cold-blooded as are the flying fishes and the flying insects : there- 

 fore I prefer the term Pterosauria to Ornithosauria. 



Never, however, have I excluded considerations from other organs 

 and functions in treating of the affinities of the Vertebrate animals 

 recent and extinct. They weighed with me, mainly, in my en- 

 deavours to interpret the nature and structure of the sacrum in 

 Megalosaurus, and the affinities so indicated. 



In Buckland's time (1824) no exception to the number two in the 

 reptilian sacrum had been recognized ; he consequently described the 

 " five anchylosed joints of the vertebral column of the Megalosaurus, as 



* " Archeeopteryx," Phil. Trans. 1863, pp. 33-47. 



t For the partial degree in which the respired or arterial blood may be so 

 transmitted from the left ventricle of the Crocodile's heart, see ' Anatomy of 

 Vertebrates,' vol. i. (1866) p. 512. 



