96 MB. n. a. seelet on the 



those of Pachyrhamplius crassirostris, appear to me to demonstrate 

 that the articulation of the ribs v/as avian, and not crocodilian. 



The sacrum differs from that of a bird chiefly in its shortness, and 

 in including but few vertebrae. Prof. Huxley has proposed to call 

 the five posterior vertebrae of the sacrum in a chicken caudal, liinit- 

 ing the term " sacral " to the five vertebrae anterior to these, while 

 the four vertebrae anterior to the latter are named dorso-lumbar. 



The number of the vertebrae is variable in these regions, and 

 they often differ slightly on the two sides of the animal. "While 

 such division on homological grounds is valuable, on morphological 

 grounds it is untenable, since at no period of the development of 

 the chick do the vertebrae named caudal and dorso-lumbar form 

 part of either the tail or back. 



The points stated by Prof Huxley serve to show probable limits 

 of variation among fossil birds, and that animals may be avian in 

 having only four or five vertebrae in the sacrum ; although they 

 would then probably belong to a new subdivision of the bird class. 

 In several Ornithosaurs there are four, five, or six vertebrae in the 

 sacrum, so that the number of vertebrae is so far in harmony with 

 the avian type as conceived by Professor Huxley. In having the 

 neural spine well developed there is a resemblance to the anterior 

 part of the avian sacrum, while in having the transverse processes 

 well developed, there is a resemblance to the hinder part of the 

 avian sacrum. The sacrum is therefore distinct from that of birds, 

 and yet altogether unlike the sacrum of any reptile. 



The caudal vertebrae vary considerably in Ornithosaurs. A.11 

 the members of the Cretaceous order Ornithocheieoidea appa- 

 rently have elongated caudal vertebrae unlike those of existing birds, 

 and resemble the anterior caudal vertebrae of reptiles in having the 

 centrum concave in front and convex behind. But, so far as I am 

 aware, in all the other forms (the Ptekodacttlia) the caudal 

 vertebrae, whether short as in Pterodactylus, or long as in Rham- 

 pTiorliynchus, have the centrum flat or slightly concave at both ends. 



In some birds the caudal vertebrae often present a marked re- 

 semblance in proportion and form of the articular face to those 

 of jPlesiosaurus and occasionally in Ornithosaurs the tail may have 

 avian proportions. But the neural arch is never so elevated as in 

 Birds, even when it is preserved ; for in some specimens from the 

 lithographic slate the neural arch in the tail is said to be absent, 

 as it is in the later caudal vertebrae of mammals. 



Thus it appears that the vertebral column shows some striking 



