156 CHELONIANS. 



tail to pass through it : moreover, all these organs can be hidden 

 within this double cuirass by means of a retractile power pos- 

 sessed by the animal. This double armour consists of a carapace, 

 or back-piece, and plastron, or breast-plate, composed of a series 

 of small bones or plates closely united together ; the first resulting 

 from the union of the sides and dorsal vertebrae, the plastron, or 

 lower buckler, being only a highly- developed sternum. These 

 organs are merely portions of the skeleton, which, in place of being 

 lodged in the depths of the soft parts, has become the superficies, 

 which is only covered by a thin, dry skin. 



This numerous and highly-interesting order of Reptiles, called 

 Chelonia, from x^ w ^> a Tortoise, are also called Testudinata, from 

 testudo, the Latin name for a Tortoise, from the double shield in 

 which the bodies of all, whether terrestrial, fresh- water, or marine 

 Tortoises, are enclosed. 



The skeleton of the Tortoise is, perhaps, the most extraordinary 

 structure with which we are acquainted. This oddly- organised 

 animal when first seen strikes the beholder with astonishment. 

 The carapace and plastron, with their connecting plates, form a sort 

 of protecting box, in which the animal lives, its head and tail ex- 

 cepted. In the land Turtles the head and feet, which are compara- 

 tively senseless, can be withdrawn within the protecting armour. 

 The ribs and sternum are both placed quite on the exterior of 

 the body, so as to form a broad dorsal shield on the upper surface, 

 and an equally strong ventral plate ; between these, the limbs and 

 the head can be more or less completely retracted. Nevertheless, 

 the modifications in the arrangement of the elements . by which 

 these changes are accomplished are of the simplest nature. In 

 the common Tortoise, Emys Europceus, the vertebrae of the neck 

 and tail being connected together in the ordinary manner, the 

 neck and caudal region of the spine present their usual flexibility, 

 but the dorsal vertebrae are strangely distorted, the upper arch 

 being disproportionately developed, while the bodies remain almost 

 in a rudimentary state ; the spinous processes of these vertebrae 

 are flattened and converted into broad osseous plates, which form 

 a longitudinal series along the centre of the back, and connected 

 together by means of sutures. The ribs are changed into broad 

 flat bones firmly united by sutures to each other, and also to the 



