1 120 CLASS REPTILIA. 



quently a parietal foramen ; the superior temporal arcade is gener- 

 ally present, but in the more specialised forms the inferior is often 

 wanting. The quadrate may be either movably or immovably 

 attached by its proximal extremity to the cranium ■ a columella is 

 very generally present ; and the ramus of the mandible never has 

 a lateral vacuity. The precoracoid appears to be fused with the 

 coracoid, and may be represented by a precoracoidal process (fig. 

 1040), or its position indicated merely by a fontanelle, while in the 

 Ichthyopterygia even the latter is wanting. A T-shaped interclavicle 

 and clavicles are present in all the earlier and a large proportion of 

 the later forms. Usually all, or nearly all, of the dorsal ribs articu- 

 late by single heads ; the dorsal vertebrae either have short or rudi- 

 mental transverse processes, which are never placed entirely on the 

 arch, or (Ichthyopterygia) a pair of facets on the centrum ; and, 

 with one exception, there are not more than two vertebrae in the 

 sacrum. Abdominal ribs are present in the earlier, but are lost in 

 most of the later forms. The humerus frequently has an ectepicon- 

 dylar foramen. The number of phalangeals in the digits of pente- 

 dactylate land forms is generally 2, 3, 4, 5, 3, in the manus, and 

 2, 3, 4, 5> 4 in the pes. 



In no case are secondary posterior nares formed by the develop- 

 ment of plates from the bones of the palate to floor the narial 

 passage ; the posterior nares consequently always forming more or 

 less slit-like horizontal apertures in the roof of the mouth. Neither 

 is the posterior extremity of the palate ever completely closed by 

 the junction of the pterygoids with the basisphenoid, after the 

 fashion obtaining in the Chelonia (fig. 1 o 1 7 bis) and some Sauro- 

 pterygia ; there being always a vacuity between the hinder extrem- 

 ities of the pterygoids which displays the presphenoidal rostrum. 



Order IV. Ichthyopterygia. — The Mesozoic Ichthyosaurs and 

 their allies were large marine Reptiles, with the body long, and 

 shaped somewhat like that of the Cetacea (fig. 1023), without 



Fig. 1023. — Greatly reduced restoration of the skeleton of Ichtliyosanrus communis; from the 

 Lower Lias of Dorsetshire. The pelvic limb is relatively too large. (After Owen.) 



either dermal or epidermal skeleton ; the limbs being modified into 

 paddles, in which the component bones were in apposition on all 

 sides, and the phalangeals were oval or polygonal, and increased 

 beyond the normal number. The skull (fig. 1024) has the facial 

 region produced into a long rostrum, mainly formed by the pre- 



