CHELONIAN REPTILES. 415 



indeed, that a great number of the peculiarities 

 which distinguish the conformation of the chelonia 

 from that of other reptiles, indicate 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, cartila- 

 ginous ; though occasionally we find them partially 

 ossified. In the tortoise, however, their ossification 

 is not only complete, but has been expanded la- 

 terally, 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, oblite- 

 rated ; and the bony fibres are continuous through- 

 out a great extent of surface. 



The most remarkable metamorphosis in the 

 osseous system of this new type is that which 

 occurs in the sternum. So expanded are all its 

 parts, that it is difficult to recognise this bone under 

 the disguised form in which it constitutes the plas- 

 tron, or broad plate, which, as we have seen, covers 

 the whole of the under side of the body. Yet, by 

 a careful examination of its structure, both in the 

 young animal, and also in the adult, when the 

 sutures are not obliterated, we may recognise the 

 nine elements of which the sternum is supposed to 



