CHELONIAN REPTILES. 467 



designed to be very moveable, we find them 

 attached either to the extremities of the transverse 

 processes, or to the articular surfaces of a single 

 vertebra ; but where solidity and security are 

 aimed at, they are always inserted between the 

 bodies of two vertebrae. This we shall find to 

 be the case also in birds, where the bones of the 

 thorax are required to be immovable. It is re- 

 markable, indeed, that a great number of the 

 peculiarities which distinguish the conformation 

 of the chelonia from that of other reptiles, indi- 

 cate an approach to the structure of birds ; as if 

 nature had intended this small group of animals 

 to be an intermediate link of gradation to that 

 new and important type of animals destined for 

 a very different mode of existence. 



The sterno-costal appendages, which connect 

 the ribs to the sternum, are, in most animals, 

 cartilaginous ; though occasionally we find them 

 partially ossified. In the tortoise, however, their 

 ossification is not only complete, but has been 

 expanded laterally, so as to form a continuous 

 surface with the extremities of the ribs and with 

 the edges of the plastron, and completely to fill 

 up the vacancy between them ; constituting a 

 dense and solid wall, which entirely closes the 

 sides of the general bony case. So strong is the 

 tendency to ossification in all these pieces, that 

 the sutures at first formed between them are 

 often, in process of time, obliterated ; and the 



