124 FISHES. 



which, before passing over the jaws, is often folded, forming 

 more or less fleshy lips. In the Sharks the skin retains its 

 external character even within the teeth, but in other fishes 

 it changes into a mucous membrane. A tongue may exist as 

 a more or less free and short projection, formed by the glosso- 

 hyal and a soft covering, or may be entirely absent. Salivary 

 glands and a velum 'palati are absent in fishes. 



With regard to the dentition, the class of Fishes offers an 

 amount of variation such as is not found in any of the other 

 classes of Vertebrates. . As the teeth form one of the most 

 important elements in the classification of fishes, their special 

 arrangement and form will be referred to in the account of 

 the various families and genera. Whilst not a few fishes are 

 entirely edentulous, in others most of the bones of the buccal 

 cavity, or some of them, may be toothed, as the bones of the 

 jaws, the palatines, pterygoids, vomers, basi-sphenoid, glosso- 

 hyal, branchial arches, upper and lower pharyngeals. In 

 others teeth may be found fixed in some portion of the buccal 

 membrane without being supported by underlying bone or 

 cartilage ; or the teeth have been developed in membrane 

 overlying one of the dentigerous bones mentioned, without 

 having become anchylosed to the Ijone. When the tooth is 

 fixed to the bone the attachment has generally been effected 

 by the ossification of the bone of the tooth, but in some fishes 

 a process of the bone projects into the cavity of the tooth ; in 

 others the teeth are implanted in alveoli. In these, again, 

 frequently a process of bone rises from the bottom, on which 

 the tooth rests. 



Many fishes, especially predatory fishes with long, lancet- 

 shaped teeth, have all or some of the teeth capable of being 

 bent inwards towards the mouth. Such " hinged " teeth 

 resume at once the upright position when pressure is removed 

 from them. They are, however, depressible in one direction 

 only, thus offering no obstacle to the ingress, but opposing the 



