DENTAL SYSTEM OF FISHES. 241 



tiilous. The myxinoids have a single pointed tooth on 

 the roof of the mouth, and two serrated dental plates on 

 the tongue. The tench has a single grinding tooth on 

 the occiput, opposed to two dentigeroiis pharyngeal jaws 

 beloAV. In the lepidosiren, a single maxillary dental plate 

 is opposed to a single mandibular one, and there are two 

 small denticles on the nasal bone. In the extinct sharks 

 with crushing teeth, called ceratodus and tenodus^ the jaws 

 were armed with four teeth, two above and two below. 

 In the himcerce^ two mandibular teeth are opposed to four 

 maxillary teeth. From this low point the number in 

 different fishes is progressively multiplied, until, in the 

 pike (Fig. 59), the siluroids, and many other fishes, the 

 mouth becomes crowded with countless teeth. 



With respect to form, it may be premised that, as or- 

 ganized beings withdraw themselves more and more, in 

 their ascent in the scale of life, from the influence of the 

 general polarizing forces, so their parts progressively de- 

 viate from geometrical figures ; it is only, therefore, in the 

 lowest vertebrated class that we find teeth in the form of 

 perfect cubes, and of prisms or plates with three sides (as 

 in myletes\ four sides (as in sca7'us\ five or six sides (as in 

 myliohates^ Fig. 60). The cone is the most common form 

 in fishes ; such teeth may be slender, sharp-pointed, and 

 so minute, numerous, and closely aggregated, as to re- 

 semble the plush or pile of velvet. These are called 

 "villiform teeth" {denies villiforines^ Lat. ; dents en velours^ 

 Fr.). All the teeth of the perch are of this kind. When 

 the teeth are equally fine and numerous, but longer, these 

 are called "ciliiform" {denies ciliiformes) ; when the teeth 

 are similar to but rather stronger than these, they are 

 called "setiform" (denies seiifonnes, Lat.; dents en brosse^ 

 Fr.); the teeth in the upper jaw of the ])ike (Fig. 59) are 

 21 



