228 THE BATH OOLITE PERIOD. CHAP. 



aeration and respiration ; but among reptiles there are various 

 degrees of incompleteness in these respects ; and the bones of 

 pterodactyles were hollow, perhaps pneumatic like those of birds *. 

 Feathers, we may truly say, accompany this high temperature in 

 birds, and seem to be essential, certainly auxiliary to it; and no 

 signs of feathers have been noticed among any of the pterosaurians, 

 while they have been seen in another fossil (archaeopteryx), of some- 

 what dubious nature, found in strata of the oolitic period. The 

 occurrence of a feather then would, according to this way of 

 reasoning, turn the balance and constitute a bird. 



Turning now to points of structure likely to be permanently 

 preserved, we may observe that birds as a rule have no true teeth, 

 and specially no such fangs as those which make the mandible of 

 rhamphorhynchus so formidable. Again, the wings of birds are 

 supported on the inner fingers of the hand m , those of the fossil 

 animals on the outer finger only. Moreover, the feet of birds which 

 have four digits differ from those of rhamphorhynchus, also having 

 four, by a curious law which seems to apply to birds and reptiles 

 generally. It consists in this : the number of joints in the toes 

 increases in both cases from the inner toe outwards, but not in the 

 same way. Thus in a bird with four toes the numbers run 2, 3, 4, 5, 

 but in a crocodile 2, 3, 4, 4 : so is it in rhamphorhynchus. 



Again, birds almost without exception have their sternal girdle 

 completed by a furcula, and none such has yet been recognized in 

 the pterosaurians. Also there is a considerable difference in the 

 form of the sternum, though in both some degree of carination 

 occurs. Lastly, in the tail of a bird the vertebrae are gathered 

 up into a short compact group, but in the rhamphorhynchi they 

 run out to an extreme length. So indeed they do in archseopteryx, 

 which is voted to have been a bird, though not well fitted for 

 flight; but had not the fine slaty stone of Solenhofen preserved 

 traces of so delicate an object as a soft feather, that admirable fossil 

 must have taken its place among the lower, though not less in- 

 teresting, nor probably less active, natural agents whose reptilian 



1 I have not satisfied myself that pneumatic foramina can be traced in the long 

 bones of Stonesfield. 



m On questions of this order in Comparative Anatomy I have the privilege of 

 unreserved communication with my colleague Dr. Eolleston. 



