vii.] THE EXTERNAL SKELETON. 241 



scutes (to facilitate motion) interposed in the mid-body 

 region. In one form, however (hlamydophorus\ bony scutes 

 are confined to the hinder region of the body, where, as 

 has been before mentioned, they coalesce with the pelvis. 

 In an extinct creature of the Armadillo kind (the Glyptodon} 

 the armour was even more complete, as there were no move- 

 able bands, but the body was invested, from the neck to the 

 root of the tail, with one solid case or carapace. 



In the Armadillos a horny epidermal skeleton is so ad- 

 justed to the bony case that the former is divisible into 

 small plates or scales corresponding with the several bony 

 scutes. 



A similar co-ordinate development of epidermal scales and 

 dermal scutes is found in some Reptiles, as in certain Lizards 

 (Cydodus), where the whole body is so doubly invested and 

 protected, as it is also in the back of the Crocodile. 



A solid investment of osseous plates may exist in Fishes, as' 

 in the bony Pike Lepidosteus and in Polypterus, but these 

 plates will be spoken of under the 

 head of dermal appendages, when 

 fishes' scales are under consideration. 



Certain Frogs (e.g. Ephippifer and 

 Ceratophrys} develop some dorsal en- 

 deronic bony plates, which become. 



connected with the underlying backbone, presenting an 

 appearance which reminds us of that extraordinary develop- 

 ment of the skeleton which we find in Tortoises. 



As in the Armadillos we find bony scutes of the exo- 

 skeleton underlying horny epidermal scales, so we find 

 beneath the tortoise-shell of Chelonians such endo-skeletal 

 scutes together with large bony plates of the endo-skeleton ; 

 only, instead of the bony and horny structures being con- 

 formable one to the other as in the Armadillos, neither the 

 number nor the outline of the bony plates of Chelonians 

 corresponds with the number or outline of their superincum- 

 bent horny scales (Figs. 204 and 207). 



Moreover in Chelonians, as noticed in describing the axial 

 skeleton, the median plates of the dorsal shield (or carapace) 

 form one with parts of the backbone, and the lateral plates 

 form one with the subjacent ribs. Besides these dorsal plates, 

 larger ventral scutes cover the under surface of the body, 

 forming what is called the plastron ; and in the Box-Tortoises 

 the ends of this plastron are movable and (the head and limbs 

 being drawn in) can be applied to the ends of the carapace, 



R 



