JAWS AND TEETH OF FISHES. 7 



two eyelids, which are anterior and posterior, like the third eyelid of birds 

 rather than the eyelids of mammals. 



The nostrils are remarkable for having a single opening in the middle 

 of the upper part of the head in the Lamprey ; but in most fishes they 

 are double, and have two openings on each side. They are placed between 

 the eye and the snout, and sometimes the two openings are different in size, 

 and one may have a slight tubular prolongation. 



On the under side of the snout sensitive tentacles are developed in the 

 Sturgeon, and similar organs surround the mouth in Cobitis, Barbels, Silurus, 

 or may occur at its angles, as in the Tench and some other fishes. 



THE MOUTH. 



The mouth is often terminal. Sometimes the jaws are unequal, and either 

 the upper or lower jaw may be the longer. When the 

 upper jaw is very short the cleft of the mouth may be 

 oblique, or even vertical, and this, with the distance to 

 which its cleft extends back, is an important character in 

 distinguishing many species. Sometimes the snout is 

 long enough to make the mouth inferior, as in some species 

 of Coregonus; though the Sturgeon is most singular in 

 the inferior and backward position of its mouth, which 

 is beneath the eyes. Lampreys are exceptional in opening the mouth from 

 side to side, as well as in the mouth being suctorial. 



The lips may te thick and fleshy, or absent. The teeth are absent in 

 Sturgeons, horny in Lampreys, but usually bony when they exist. There 

 are no teeth in the mouth in fishes of the Carp family, but the pharyngeal 

 bones are well developed and carry teeth,^ which are arranged in parallel series, 

 of which there may be one, two, or three. This character is so important that 

 it is always stated numerically in a formula thus, 5'4 4'5, which means five 

 teeth in the long row, and four in the short inner row on each side. 



The teeth are often remarkably developed (Fig. 3), and may be present not 

 only upon the pre-maxillary and maxillary bones, and mandible, but also upon 

 the vorner, palatine, and pterygoid bones, and upon the tongue. There are 

 often teeth of different sizes in the same jaw, as in the lower jaw of the Pike. 

 They are frequently fine, dense, and villiform, as in the Perch. Occasionally 

 the teeth may be in a double row, as in the upper jaw of the Flounder. 

 The jaws are sometimes protractile. 



THE OPEUCULUM AND GILL-ARCHES. 



The bones which form the gill-covers, or are connected with the branchial 

 apparatus, furnish important distinctive characters for genera and species. 



