PHYLUM VIII VERTEBRATA 143 



cheek plates is pierced by a lateral temporal vacuity, leaving a narrow bar 

 above and another narrow bar below. Among Lacertilians this lower bar 

 becomes lost, and finally among . Snakes both bars disappear, leaving but one 

 extensive vacuity. For those orders of reptiles distinguished by having two 

 temporal arches, namely, the Ehynchocephalians, Crocodilians, Dinosaurs, and 

 Pterosaurs, Cope has projDosed the term Archosauria. This group properly 

 includes also the Squamata, in which one or both arches have become obsolete. 



On the under side of the skull the pterygoid forms the connection 

 between quadrate, basisphenoid, and palatines. It varies considerably in size 

 and shape amongst the different orders, according as the quadrate and maxillo- 

 palatine apparatus are movably or rigidly united with the wall of the skull. . 

 In many forms a transverse bone (ectopterygoid) is developed between the 

 pterygoid and maxilla, and a slender column-like bone called the columella or 

 epipterygoid, extends between the paxnetal and pterygoid (Fig. 253). Some- 

 times a flat expansion of the maxillae joins with the palatines, and enters 

 with these bones, the vomers, premaxillae, and pterygoids into the formation 

 of a secondary bony palate. The lower jaw is composed, as in Amphibians, 

 of a dentary, splenial, coronoid, angular, and surangular foi'med from mem- 

 brane ossifications, and an articular formed from Meckel's cartilage. Crocodiles 

 have the lower jaw as well as many of the cranial bones hollow and pneu- 

 matic. 



Teeth are wanting only among Chelonians and isolated representatives of 

 other groups. As a rule they occur extensively on the jaw bones, and in 

 some cases also in the palatines, pterygoids, and vomers. The greater number 

 are single-rooted, those with divided roots being very exceptional. The 

 crown consists chiefly of dentine, covered with a layer of enamel ; cement 

 enters but slightly into its composition, and vasodentine is entirely wanting. 

 The form is usually sharply conical, either erect or recurved, but there are 

 many other varieties, such as depressed, hemispherical, or even pavement-like ; 

 some have bevelled edges, or are chisel- or spade-shaped, and a few are multi- 

 cuspidate. Pleodont teeth have solid crowns, as contrasted with coelodont, 

 which grow from persistent pulps. Thecodont or socketed teeth are inserted in 

 alveoli, acrodont fused with the supporting bone along the outer rim or top, 

 and pleurodont teeth are developed laterally along the flange-like inner rim of 

 the jaw. 



With the exception of Snakes and certain Lacertilians, two pairs of limbs 

 are always present. In the j^edoral arch the principal bones are the coracoid 

 .and scapula, the former usually exceeding the latter in size, and forming 

 either alone or in connection with the scapula the glenoid cavity for the 

 humerus. Clavicular and interclavicular elements may or may not be present, 

 and the same is true of the precoracoid, which occurs usually as an anteriorly 

 directed process of the coracoid. The scapulae are normally directed upward 

 and backward, and are well separated by the clavicular arch when present. 

 The interclavicle (or episternum), may be rhomboidal, cruciform, or T-shaped, 

 and difi"ers from the adjacent bones in being of membranous origin. Posterior 

 to the interclavicle is usually a flat, rhomboidal, or aspidate sternum, to which 

 the anterior dorsal ribs are attached by means of intermediate pieces. Very 

 frequently the sternum is wanting ; when present its antero-lateral margin is 

 always applied against the coracoid, and when absent the coracoids unite in a 

 imedian symphysis or are joined together by ligaments. 



