214 CHARACTERS OF VERTEBRATE ANIMALS 



not only horny plates and scales of epidermal nature, but also 

 bony pieces developed in the deeper part of the skin both above 

 and below. Here, however, these pieces cannot be stripped off 

 with the skin, but are to a great extent intimately welded to 

 parts of the internal skeleton. The arched upper or dorsal half 

 of the shell is termed the carapace, and the flat under or ventral 

 piece with which it is firmly united at the edges is the plastron. 

 It will be convenient first to deal with the superficial horny 

 shields and then with the underlying bony plates. Beginning 

 with the carapace, there is a series of neural shields running down 

 the centre, and these are flanked on either side by broad trans- 

 verse costals, outside which again are more numerous marginals, 

 forming a right and left series separated in the extreme front 

 by a median neck-shield (nuchal) and behind by a pair of 

 caiidals. Some of the marginals on either side bend sharply 

 round to the under surface and help to cover the bones of the 

 plastron. The greater part of this, however, is veneered by six 

 pairs of horny shields, of which those in front (gulars) and 

 behind (anals) are smaller than the rest, which have received 

 the names of humerals, pectorals, abdominals, and femorals. 



When these horny shields are stripped off, a number of 

 bony plates are exposed which have a very similar arrange- 

 ment, though their number is not the same, and therefore, of 

 course, their edges do not correspond with those of the over- 

 lying plates, as is the case with the armour of caimans. 

 Running down the middle of the carapace are eight neural 

 plates, which may be regarded as the flattened tops of vertebrae, 

 and which are continuous on each side with a similar number 

 of costal plates fused with the underlying ribs. The carapace 

 is completed by a nuchal plate in the middle of its front margin 

 and pygal plates similarly placed behind, while a series of eleven 

 marginal plates are disposed on either side. The plastron is 

 made up of one unpaired plate, and four others arranged in 

 pairs. The former is possibly to be compared with the inter- 

 clavicle of a lizard (see p. 198), while the others are in all 

 probability to be regarded as equivalent to the abdominal ribs of 

 the crocodile. 



As will have been gathered from the foregoing description, 

 there is intimate union between bony plates belonging to the 

 external skeleton and parts of the internal skeleton, i.e. the 



