SQUAMATA 139 



as in most other reptiles; they often bear teeth, a primitive char- 

 acter. The teeth of all living lizards and snakes are not inserted 

 in sockets, as are those of the crocodiles, but are co-ossified to the 

 margins or sides of the jaws or the bones of the palate. But this 

 is probably not a primitive character; doubtless the teeth of the 

 early lizards were inserted in sockets like those of most other rep- 

 tiles. The shoulder bones are absent in many and vestigial in 

 some others. When present and fully developed, they comprise 

 the shoulder-blades or scapulae, a single coracoid on each side, 

 the clavicles, and an interclavicle. The vertebrae, except in some 

 lizards, are procoelous, that is, with the body concave in front and 

 convex behind, a peculiar structure that was developed only in 

 crawling animals. In addition to the usual articulations for the 

 union of the vertebrae there are also, in some of the lizards 

 and mosasaurs and all of the snakes, additional ones called the 

 zygosphene and zygantrum, which will be best understood by 

 reference to Fig. 12, p. 28. But little less characteristic than 

 the loose articulation of the lower jaws, so unique in this 

 order of reptiles, is the manner of attachment of the ribs. 

 They are always single-headed, articulating only with the body 

 or lower part of the vertebra. The single-headed ribs of the 

 plesiosaurs articulate with a projection on each side of the 

 arch of the vertebra; those of the turtles to the space between 

 the adjacent vertebrae; nearly all other reptiles have double- 

 headed ribs, articulating in various ways. This character, it is 

 seen, though apparently a simple one, immediately distinguishes 

 a lizard or a snake from all other animals, except the thalattosaurs 

 and protorosaurs. 



There is much difference of opinion among naturalists as to the 

 proper classification of the different groups of this order of reptiles. 

 Usually it is divided into four suborders, the Lacertilia or lizards; 

 the Dolichosauria or long-necked lizards of the past; the Mosa- 

 sauria, or extinct swimming lizards; and the Serpentes or Ophidia, 

 the snakes. It matters very little which classification one accepts 

 so long as it is remembered that the first three groups are closely 

 related to each other. 



