xiii PHYLUM CHORDATA 315 



that of a bird in its general shape and in the obliteration of the 

 sutures. There is a ring of sclerotic bones. The quadrate is im- 

 movable. There is a sternum. The fore-limbs are modified to act 

 as wings by the great enlargement of the post-axial digit, for the 

 support of a fold of skin. The posterior limbs are weak and have 

 four or five digits. The teeth are implanted in sockets. In the 

 1 train the optic lobes were widely separated by the cerebellum, and 

 the latter bore a pair of lateral processes or flocculi (Fig. 959). 



Systematic Position of the Example. 



There are twenty known species of the genus Lacerta, occurring 

 in Europe, Asia, Africa, and North America. Lacerta is a member 

 of the sub-order Lacertilia of the order Squamata. The flattened 

 and elongated tongue with notched apex places it in the section 

 Leptoglossse of that sub-order. Among the Leptoglossae the 

 family Lacertida?, which comprises Lacerta and a number of other 

 genera, is characterised by the presence of dermal bony supra- 

 orbital and supra- temporal plates, by the presence of small granular 

 or wedge-shaped scales, and of pleurodont conical teeth, excavated 

 at the base. The chief distinctive marks of the genus Lacerta are 

 the presence of comparatively large shields on the head and on the- 

 ventral surface, the arrangement of the scales of the trunk in 

 transverse rows which become circular zones or rings on the tail, 

 the development of a collar-like band of larger scales round the 

 neck, and the laterally-compressed falciform claws, grooved on the 

 lower surface. 



3. GENERAL ORGANISATION OF RECENT REPTILIA. 



External Features. In external form, as in some other 

 respects, certain of the Lacertilia exhibit the least specialised 

 condition to be observed among the living Reptilia. Lacerta is 

 such a central type, and the general account of that Lizard which 

 has just been given applies in all the points of cardinal importance 

 to a large proportion of the Lacertilia. Modifications take place, 

 however, in a variety of different directions. Of such the following 

 arc a tow of the chief. The tail region is usually, as in the example, 

 extremely long and tapering ; but in some groups of Lizards it is- 

 comparatively short and thick; and in others it is depressed and 

 expanded into a leaf-like form. In the Chamaeleons the long and 

 tapering tail is used as a prehensile organ, the coiling of which 

 round branches of the trees in which the animal lives aids in 

 maintaining the balance of the body in climbing from branch to- 

 branch. 



In the limbs there is likewise a considerable amount of variation 

 in the different groups of the Lacertilia. Moderately long penta- 

 dactyle limbs like those of Lacerta are the rule. In the- 



