IO48 CLASS REPTILIA. 



introductory chapter (p. 904). The bones of the skull are of a 

 dense ivory-like structure, and in most cases their sutures persist ; 

 although in certain groups the premaxillae, frontals, and parietals 

 may respectively unite. A new element — the transverse, or trans- 

 palatine, bone (fig. 1089, Ts) — connecting the maxilla with the 

 pterygoid, appears ; but this is absent in the Chelonia and some 

 Ophidia. An epipterygoid, or columella, connecting the pterygoid 

 with the parietals, may also be present. A parietal foramen may or 

 may not be present ; and the occipital condyle is usually placed at 

 the hinder extremity of the cranium. The apertures of the external 

 nares may be terminal, as in the Amphibia, or approximated to the 

 orbits, as in the Birds ; and the bones of the palate may develop 

 a floor underlying the narial passage and thus cutting it off from the 

 mouth. As a general rule the mandibular symphysis is not an- 

 chylosed ; but this takes place in the Chelonia and Ornithosauria. 

 The quadrate may be either loosely or immovably attached to the 

 cranium. 



The dentition is usually well developed, and the teeth of different 

 parts of the jaws are occasionally more or less differentiated ; although 

 there is no known instance where they are implanted by double roots, 

 or where their crowns have deep infoldings of enamel. They may be 

 present not only in the jaws, but also upon the palatine, pterygoid, 

 and more rarely the vomer. In other instances, however, teeth may 

 be entirely wanting, and the jaws simply ensheathed in horn. The 

 teeth may be anchylosed to the outer side of the jaws, when the 

 dentition is termed pleurodont (fig. 975) ; or to their summits, when 



the term acrodont is ap- 

 plied ; or they may be 

 set in a groove, with or 

 without anchylosis to 

 the bone ; or, finally, 

 they may be placed in 



Fig. 975-— Inner view of the left ramus of the mandible of distinct SOCketS when 



the dentition is said to 

 be thecodont. The teeth on the palate are generally anchylosed to the 

 subjacent bones. There is usually a continuous succession of teeth 

 developed throughout life ; the new teeth coming up beneath those 

 in use and absorbing the base of the crown, as is shown in the teeth 

 of the Gharial represented in fig. 1090. In shape the teeth of 

 Reptiles present great variation ; but a very common type, from 

 which many of the variations are derived, consists of a more or less 

 laterally-compressed and recurved cone, with fore-and-aft cutting 

 edges, or carince, which may or may not be serrated (fig. 976). Oc- 

 casionally, however, the teeth of the jaws or those of the palate 

 may have nearly flat crowns adapted for crushing (fig. 986). 



