THE BRITISH REPTILES. 107 



Of the three snakes, one, the Viper, is the sole representative of 

 a family in which the upper jaw is toothless except for the fangs and 

 their reserves, and in which the hinder upper jawbones can be 

 vertically raised ; and the Viper can be further distinguished from its 

 two compatriots by having its labial shields separated from its eye 

 by scales. In the Ringed Snake and Smooth Snake the upper jaw 

 is fully toothed, the hinder upper jawbones are fixed horizontally, 

 and the labial shields extend right up to the eye without any 

 intervening row of scales. Thus we can 1 finish this sorting out of 

 our reptilian genera with 



OPHIDIA 



1. (Colubridae) 



Both jaws fully toothed ; hinder upper maxillarie-s fixed 

 horizontally ; labial shields extending to eye. 

 Scales keeled, Tropidonotus, 79. 

 Scales smooth, Coronella, 80. 



2. (Viperidae) 



Upper jaws toothless except for the fangs and their 

 reserves; hinder upper maxillaries capable of vertical 

 erection ; labial shields separated from the eye by 

 scales. Vipera, 81. 



But useful as this may be as a ready means of identification, 

 something more is needed by way of description, and that can 

 best be given in a more formal way. Let us then begin again at 

 the beginning and work our way down from the class to the families. 

 What are reptiles ? Reptiles are cold-blooded vertebrates clothed 

 with scales or scutes instead of hair or feathers. They breathe by 

 lungs throughout life, and their skull articulates with the backbone 

 by a median occipital condyle, and their lower jaw articulates with 

 the skull by a quadrate bone. They are grouped in ten sub-classes, 

 of which only two, Chelonia and Sauria, are represented in the 

 British list of living animals, the Chelonians owing their place 

 therein to a few drifted specimens of only two species. 



The characteristics of the Chelonia are briefly as follows. They 

 are oviparous reptiles with a broad, flat, compact body encased 

 within a bony shell of dermal bones, of which the dorsal series is 

 known as the carapace and the ventral as the plastron. The cara- 

 pace is formed of the neural spines of the vertebrae, of the expanded 

 ribs, and a series of marginal plates round the outer edge ; the plastron 

 is formed of the clavicles and a few other elements. The carapace 

 and plastron may or may not be covered with horny plates known 

 as tortoiseshell, in which many nerves terminate. The quadrate is 

 firmly joined to the skull and does not move. There is no sternum. 

 The ribs have their capitular portions only ; there are two sacral 

 vertebras. The jaws have cutting horny sheaths and no teeth, 

 except in a rudimentary state in the embryo. The sub-class is 

 made up of two orders, Atheca and Thecophora, 



