412 



CHELONIA. 



the animals were not then existing, it may be mentioned that 

 the earliest known Chelonians were in every way as specialised 

 as those now living. 



Sub-order 1. ATHECAE. Vertebrae and ribs free, not fused 

 with the carapace, which consists of numerous juxtaposed poly- 

 gonal plates. The skin is leathery without horny plates. There 

 are eight plastral elements in the ventral part of the shell. 

 The parietal bones are without descending processes, and nearly 

 cover the supraoccipital ; the temporal fossa is roofed in. Neuro- 

 central suture on all the vertebrae except the posterior caudal, 



the cervicals are short and the head 

 is not retractile. The limb-girdles 

 are essentially like those of other 

 Chelonians ; limbs paddle-shaped, 

 clawless ; digits of fore-limb elon- 

 gate. Marine, within the tropics of 

 the Indian, Atlantic, and Pacific 

 Oceans. 



c' 



Flo. 232. Heart and great arteries 

 of a Cheloniann (Chelydra), (from 

 Gegenbaur). d right, s left auricle; 

 c carotid ; ad right, ps left aortic 

 arch ; pd right, ps left pulmonary 

 artery ; c.' coeliac artery ; ad right, 

 ss left subclavian artery. 



The psdigree of this sub-order has been 

 much disputed. As an indication of the 

 slight value of all such speculations, it 

 may be mentioned that it has by some 

 authorities (Cope, Dollo, Boulenger) been 

 regarded as the most primitive of recent 

 Chelonia, by others (Baur, Dames, Case) 

 as the most specialised. Dermochelys 

 Blainv. (Sphargis Merrem) ; only one 

 species. D. coriacea L. reaches 6^ ft., 

 small specimens (to 3 in. ) and large 

 specimens only known breeds on sandy 

 shores, flesh unwholesome. Fossil forms 

 from the Upper Cretaceous and Ter- 



tiaries, Prolostegz. Cope, Prctoftphargis Capellini, Psephophorus H. v. 



Meyer, Eosphargis Lyd. 



Sub-order 2. THECOPHORA. Thoracic vertebrae and ribs im- 

 moveably united with a series of median (neural) dermal plates 

 and a paired series of lateral (costal) dermal plates. Parietals 

 prolonged downwards meeting the pterygoids or separated from 

 them by an interposed epipterygoid. 



Sectional. Cryptodira. 



Carapace covered with horny shields ; neck bending in a sigmoid curve 

 in a vertical plane ; pelvis not fused with the carapace. 



