Characteristics of the Vertehrata. Ivii 



in Squaviata, but are persistent in Loricata. The number of 

 the vertebrae may amount to several hundreds in the Ojihidia, 

 and in the Saurian Awpldshaenoidea may be as many as one 

 hundred and thirty. Though ordinarily the number is much 

 smaller in Reptiles provided with limbs, even in the non-sei-penti- 

 form Monitor it may be no less than one hundred and forty. There 

 are two cervical vertebrae in Ophidia ; in Saiiria their number may 

 amount to ten ; in the Loricata it is usually eight. The dorsal 

 vertebrae are movably articulated with each other, except in 

 Chelonia ; the sacral vertebrae are seldom more than two in 

 existing Reptiles, though this number was often much exceeded in 

 extinct forms of the Class. Lumbar vertebrae do not exist in 

 Ojihidia ; they are present in Chelonia, and in number from four 

 to five, whilst in Sauria they are reduced to two, or even one. 



The shoulder girdle is entirely absent only in Opliidia^ but 

 present in a rudimentary condition in the serpentiform Sauria. 

 The clavicle is wanting in Loricata, Chamaeleonoidea, and Sau- 

 ropterygia. The sternum is wanting in Ophidia^ Chelonia^ and in^ 

 some of the serpentiform Sauria. The ribs in the Chelonia form 

 by fusion with exoskeletal ossifications the expanded lateral or 

 ^ costal ■* plates of the carapace. The sternum is absent, and the 

 ventral plates, constituting the ' plastron,^ are exclusively dermal 

 ossifications. In the CrocodiUna^ the ribs of the anterior thoracic 

 vertebrae, and, with the exception of those belonging to the atlas, 

 of the cervical also, articulate with their respective vertebrae by 

 two separate processes, the * tuberculum,' and the ' capitulum.' 

 The ribs of the Sauria have only a single articular facet, which, 

 however, may show a tendency to bifurcate. The ribs carry pro- 

 cessus iincinati in the Crocodiles and in Hatteria. Many Reptiles 

 have free abdominal ribs, which may be either true endoskeletal 

 elements, or ' parostotic ' ossifications of intermuscular fibrous septa, ^ 



or, as in Hatteria, of the subcutaneous fibrous mesh. In Ophidia, 

 the posterior pair of limbs is sometimes represented by a jiair of 

 small bones placed anteriorly to the anus ; and in the serpentiform 

 Sauria the pelvic girdle and its appendages are represented merely 

 by a single iliac bone, attached on either side to a single ' sacral ' 

 vertebra. But in all other Reptiles the three pelvic bones are 

 present, forming pubic and ischiac arches by abutment upon the 

 ossa ilii, which do not extend forwards anteriorly to the acetabulum. 



