THE TEETH OF FISHES. 215 



destitute of true calcified or dentinal teeth, the armature of 

 the mouth consisting of horny cones or serrated plates. 



,The parasitic Myxine, which is found in the interior of 

 other larger fish, is furnished with a median curved conical 

 tooth, of horny consistency, which is believed to act as a 

 holdfast, while the serrated edges of the horny plates upon 

 the tongue are brought into play in boring a way into the 

 interior of its host. 



The concave circular disc which surrounds the mouth of 

 the Lamprey is covered with concentrically disposed horny 

 teeth, of simple conical form ; in addition to these there 

 are lingual and palatal horny plates. 



The teeth of Elasmobranchii present rather more of sim- 

 plicity and uniformity of plan than do those of most fish, 

 and it will hence be convenient to describe their teeth first, 

 although in most respects they stand at the head of the 

 class of fishes, and present many indications of affinity with 

 the Batrachia. 



In the Plagiostomi the mouth is a transverse, more or less 

 curved fissure, opening upon the under surface of the head 

 at some little distance behind the end of the snout. Hence 

 it is that a shark in seizing its prey turns over upon its back 

 or at all events upon its side. 



The jaws, which are made up of the representatives of the 

 palato-quadrate arch, and of Meckel's cartilage, neither true 

 maxillse nor premaxillae being present, are cartilaginous in 

 the main (although covered with a more or less ossified crust), 

 and therefore shrink and become much distorted in drying. 

 The shape of the jaws differs in the various groups of 

 Playiostomi, in some each of the two jaws being a tolerably 

 perfect semicircle, while in others they are nearly straight 

 and parallel to one another (see fig. 92 and fig. 96); but in all 

 the rounded working surface of the jaw is clothed or encased 

 by teeth, which are arranged in many parallel concentric 

 rows. 



