^^O CLASSIFICATION OF VERTEBRATES 



of the skull, as described later in this Chapter (p. 576) and again 

 into several orders. The only orders with extant species are the 

 Chelonia (tortoises and turtles), the Rhynchocephalia (containing 

 only Sphenodon from New Zealand), the Squamata (lizards and 

 snakes), and the Crocodilia. Great Britain has only six species, 

 all Squamata. Lacerta vivipara is the common lizard, L. agilis, the 

 sand lizard, which is of very local occurrence, and Anguis fragilis, 

 the slow or blind worm, which is legless and snake-hke ; there are 

 also three snakes, Vipera herns, the adder, Natrix natrix, the grass 

 snake, and Coronella aitstriaca, the smooth snake which is rare and 

 local. Contrary to popular belief Ireland is not free from reptiles, 

 for the common lizard is found there. 



CLASS VI — AVES 



The birds are warm-blooded, flying amniotes with feathers. 

 Except for some extinct forms they have lost their teeth, but 

 in the skeleton and soft parts generally they differ little from 

 reptiles. There are scales on the legs, the circulation is reptilian, 

 and they lay large yolky eggs, which are, however, much more 

 cleidoic, or shut off from the environment, than those of reptiles. 

 There are a number of adaptive features connected with flight, 

 and a great development of instincts, such as those of migration, 

 territory holding, nest-building and song, which are ancillary 

 to reproduction. The classification of birds is difficult, partly 

 because they are so uniform and partly for lack of fossils. Most 

 of the birds of the world, and all the British species, belong to the 

 super-order Neognathae. The remainder, the penguins, ostriches 

 and some others, are all flightless, but most are not closely related. 

 The ostriches and similar running birds (the former Ratitae) are 

 now by some authors included in the Neognathae. 



CLASS VII — MAMMALIA 



The mammals have already been considered in Chapter 25. 



STRUCTURE AND EVOLUTION OF VERTEBRATES 



The types which we have considered in earlier chapters fall 

 fairly easily into a rough series, the members of which show 

 increasing complexity. The earliest chordate was probably a 



