86 MB. n. G. SEELEY ON THE 



a single aortic arch, and remarkably modified respiratory organs ; 

 but it is, to say the least, highly probable that the Pterosauria, 

 if not the Dinosauria, shared some of these characters with them. 

 The amount of work involved in sustaining a Pterodactyle in the 

 air would seem, physiologically, to necessitate proportional oxida- 

 tion and evolution of waste products in the form of carbonic acid. 

 If so, a proportional quantity of heat must have been evolved, and 

 there must have been a ready means of eliminating the carbonic 

 acid from the blood. We know of no such means except those 

 which are afforded by highly developed circulatory and respiratory 

 organs ; and therefore it is highly probable that the Pterodactylea 

 had more perfect organs of this kind than their congeners, accom- 

 panied by the correlative hot blood. But since we know that the 

 organs of respiration and circulation of a bat are very different 

 from those of a bird, it is quite possible that those of a Pterodac- 

 tyle may have been different, in detail, from either "*. 



This passage may perhaps be reconciled with the preceding one 

 by means of the dictum often laid down by Professor Huxley, that 

 birds are greatly modified reptiles. But I do not think that the 

 assertion that birds are reptiles would go a great way towards 

 aendering it probable that the pneumatic skeleton of Pterodac- 

 tyles is an adaptive modification. 



On the hypothesis that Pterodactyles are reptiles, Professor 

 Huxley would infer, I think, that flight caused the development 

 in them of the pneumatic skeleton ; but seeing that the Cheiro- 

 ptera, among mammals, have great powers of flight without the 

 skeleton being pneumatic, the statement can but rank as a sur- 

 mise unsupported by evidence, and, so far, contrary to evidence. 

 Until some living animal, demonstrably reptilian, is discovered to 

 possess limb-bones marked with pneumatic foramina, it seems to 

 me that teaching from any one wiU lack weight when it refers 

 fossil pneumatic skeletons to the Eeptilia. Among living animals 

 the pneumatic foramina exist only that avian lungs may have their 

 air-cells prolonged into the bones ; so that no other function can 

 fairly be inferred for them when they are found in fossil bones. 



The pneumatic foramina of Ornithosaurs so closely resemble 

 those of birds in almost every bone of the skeleton, that the re- 

 semblance often amounts to complete coincidence. The holes are 

 usually in exactly the same positions on each of the bones in both 

 groups ; and in both they have the same details of reticulate 

 structure. It must, then, be sound physiology to infer that such 

 P, Z. S, 1867, pp. 417-18. 



