14^ 



CHAPTER IV. 



CHELONIANS, OR SHIELDED REPTILES. 



[" The body," as described by Dr. Gray, " is covered with square 

 ■ embedded plates, generally forming a dorsal and ventral shield 

 united by their margins, leaving only the head, neck, limbs, and 

 tail free, and (in some species, as the Box Tortoises, Cinastemon, 

 shut up by movable closely-fitting doors) only covered with a scaly 

 skin ; the upper shield formed of the ribs united together and 

 adherent to the dorsal vertebrae by a toothed suture, and surrounded 

 by a series of bones forming the edge of the shields ; the lower 

 shield or sternum, formed of four pairs and a central anterior, 

 bone ; the jaws toothless, covered with a horny bill, rarely hid by 

 fleshy lips ; eyelids distinct ; drum of the ear visible ; legs short 

 and thick ; tail conical." " The natural dwelling chamber of the 

 Chelonia consists chiefly," says Professor Owen, " and in the 

 marine species (Chelone) and Mud Turtles (Trionyx) solely, of the 

 floor and the roof; side walls of variable extent are added to the 

 fresh-water species (Emydians) and Land Tortoises ( y Testudi?iians). 

 The whole consists of ' osseous plates,' with superincumbent horny 

 plates, or ' scutes,' except in the Soft or Mud Turtles (Trionyx 

 and Sphargis), in both of which these are wanting.* 



These animals, to which a portable stronghold is thus given 

 in compensation for inferior powers of locomotion and defence, 

 are recognisable at a glance from the singular armour with which 

 Nature has provided them. A double shield envelopes all parts 

 of their bodies, only permitting the head, neck, legs, and tail to 

 pass through it; moreover, all these organs can be hidden within 

 this double cuirass by means of a retractile power possessed 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, 



* " Circle of the Sciences." 



