170 TEE ANATOMY OF VERTEBRATED ANIMALS. 



a. A pectoral arcli and urinary bladder. 



III. — Lacert'dia. 

 h. No pectoral arch, and no urinary bladder. 



IV. — Ophidia. 



b. The dorsal vertebrae have double tubercles in the place of transverse 

 processes {^Perosjjondylia). The limbs are paddle-hke. 



y. — IcJdIu/osaicria. 



C. The anterior dorsal vertebrae have elongated and divided transverse 

 processes, the tubercular being longer than the canitular division 

 ( Suchospondylia). 

 a. With only two vertebrae in the sacrum. 



VI. — Crocodilia. 



h. With more than two vertebrae in the sacrum. 

 a. The manus without a prolonged ulnar digit 



1. The hind-limb saurian. 



VII. — Dicynodontia. 



2. The hind-limb ornithic. 



VIII. — Oryiitlioscelida. 

 h. The manus having an extremely prolonged ulnar digit. 



IX. — Pterosauria. 



I sliall describe the exoskeletal, endoskeletal, and dental 

 systems of the chief groups of the Heptilia., under the several 

 heads here enumerated, and I shall then give an account of 

 these systems in Aves. But modifications in the myology, 

 neurology, splanchology, and development of the two classes 

 may be conveniently considered together, under those several 

 heads, in another chapter. 



1. The Chelonia. — The Tortoises and Turtles are those 

 reptiles which most nearly approach the AiJiphibia, though 

 they depart very widely not merely from the amphibian, but 

 from the ordinary vertebrate type, in some respects. 



A horny epidermic exoskeleton is absent in the soft Tor- 

 toises [Trio7iyx)^ the bodies of which are covered by a soft 

 integument ; but, in the other Chelonia^ the epidermis is modi- 

 fied into horny plates, which constitute the so-called " Tor- 

 toise-shell," and have in general a very definite arrangement. 

 Tlje dorsal surface of the body presents three series of central 

 I'lates, of which five are in the middle and four upon each side 

 (4, 5, 4). The margins of the dorsal shield are guarded by 

 tweritj'-four or twenty-five plates ; one on the middle line in 

 front, called nuclial ; one or two behind, pygcd j and eleven 



