xiii PHYLUM CHORDATA 327 



are lodged in sockets, and usually have compressed crowns 

 (Fig. 1027). 



ORDER IX. PTEROSAURIA. 



Extinct Reptiles, the structure of which is greatly modified in 

 adaptation to a flying mode of locomotion. The vertebrae are 

 proccelous, the neck elongated. The sacrum contains three to six 

 vertebrae. The anterior thoracic ribs are bifid. The skull resembles 

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

 sutures, but not in more essential features. There is a ring of 

 sclerotic bones. The quadrate is immovable. There is a sternum. 

 The fore-limbs are modified to act as wings by the great enlarge- 

 ment 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 brain the optic lobes were 

 widely separated by the cerebellum, and the latter bore a pair of 

 lateral processes or flocculi (Figs. 1029-1031). 



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 

 Leptoglossae of that sub-order. Among the Leptoglossae the 

 family Lacertidae, 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 

 are a few of the chief. The tail region is usually, as in the example, 

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



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