GOOD OPENING FOR A YOUNG DENTIST. 43 



pharyngeal bones. In certain genera they exist on 

 all those parts ; while in others they are wanting 

 on some, or are even entirely absent on all. The 

 denominations of the teeth are derived from their 

 position, that is from the bones to which they are 

 attached, and are consequently as numerous as the 

 varieties of their situation. In the upper portion 

 of the mouth of a trout, for example, there are five 

 rows of teeth. The single middle-row is placed 

 upon the central bone of the mouth called the 

 vomer ; a row on each side of it is fixed on the right 

 and left palatal bones, while the outer-rows or 

 those of the upper -jaw, properly so called, are 

 situate on the maxillary bones. In the under por- 

 tion of the mouth there are four rows, that is, one 

 on each side of the tongue, and another external to 

 these on each side of the lower-jaw. As to the 

 form of teeth in fishes, the majority are hooked and 

 conical, and more or less acute. 



In the majority of osseous fishes, besides the lips, 

 which, even when fleshy, having no peculiar muscles, 

 can exert but little strength in retaining the ali- 

 ments, there is generally in the inside of each jaw, 

 behind the anterior teeth, a kind of membranous 

 fold or valvule, formed by a replication of the interior 

 skin, and directed backwards, of which the effect is 

 to hinder the alimentary substances, and especially 

 the water gulped during respiration, from escaping 

 again by the mouth. This structure does not, as 

 formerly supposed, constitute a character restricted 

 to the genus Zeus, but exists in an infinity of 

 fishes. 



