2o6 CHARACTERS OF VERTEBRATE ANIMALS 



made out, and the upper surface is sculptured something like 

 the scutes. It is, indeed, almost certain that some of the flat 

 bones which help to cover in the skull in those vertebrates 

 possessing a bony skeleton are in reality scutes which have sunk 

 inwards and attached themselves firmly to underlying parts. 



The bones of the jaw are very long, and their margins 

 present a large number of sockets, arranged in a single row, for 

 the reception of the formidable pointed teeth. As in a bird or 

 lizard, the lower jaw hinges on behind to an elongated bone 

 known as the quadrate (see p. 143); but whereas in those forms 

 the bone in question is more or less movable so that a sort 

 of double jaw-joint is constituted, here it is firmly fixed to the 

 skull. Such a double joint detracts somewhat from firmness, 

 which is very necessary to an animal that, like a crocodile, has 

 to overcome prey possessing considerable muscular strength. 

 And further, the long jaws enable the mouth to be opened 

 widely enough without the presence of a joint of the kind. A 

 notable peculiarity is found in the very great length of the bony 

 palate which supports the roof of the mouth, and above which 

 runs a passage into which the cavities of the nose open, and 

 along which air is conducted to and from the glottis, or aperture 

 of the windpipe. In a bird or lizard the internal openings of 

 the nasal cavities are situated on the roof of the mouth fairly 

 far forwards, but in the crocodile the arrangement described 

 throws them very far back, and as the top of the windpipe 

 projects into them a distinct air-passage from the external nos- 

 trils backwards is constituted, so that breathing can take place 

 when the tip of the snout is above water, even if the mouth 

 be open, without any danger of water getting down into the 

 lungs. Similar anti-choking arrangements are found in other 

 air-breathing aquatic vertebrates, and will be described in another 

 chapter. 



The thoracic ribs exhibit one feature which is characteristic 

 of birds, each of them having a backwardly-directed plate on 

 its margin not very far from the backbone (see p. 145). These 

 uncinate processes add to the firmness of the wall of the chest. 

 Not only are ordinary ribs present, but also " abdominal ribs ", 

 situated behind the sternum and developed in connection with 

 the muscles. It is interesting to note that ribs of the kind are 

 also present in the oldest known fossil bird {Arckcsopieryx), 



