42 



CLASSIFICATION OF FISHES. 



is the inter -operculum (c), because it fills up the interval 

 between the base of the pre-operculum and the sub- 

 operculum. These distinctions will be rendered more 

 intelligible by the annexed figure^ which represents the 



head of the common tench. 



These bones are either 

 smooth both on their outer 

 surface and their edges^ or 

 they are armed^ in one or 

 both situations, either with 

 spines or prickles, or very 

 fine dentations like the 

 teeth of a saw ; and these modifications are of great 

 importance in determining generic groups. The hinder 

 margin of the entire operculum, or gill-cover, is gene- 

 rally bordered by a thin membranaceous skin_, for the 

 purpose of closing the opening of the gills more ef- 

 fectually : this skin is a continuation of that which 

 supports the branchial rays^ and these latter commence 

 at the bottom of the head, adjoining the throat; and the 

 number of these rays are considered indicative of ge- 

 neric peculiarities. In many groups which possess gill- 

 covers, the plates are either immoveable, or are so com- 

 pletely concealed under the skin that they are not to 

 be detected except by dissection. In such instances, 

 the aperture becomes so small as to be analogous to the 

 spiracles of the cartilaginous tribe ; it assumes, in 

 fact, the appearance of a slit, and is then termed a 

 spiracle. This character pervades the whole of the 

 aberrant tribes of our present arrangement, and even 

 extends to such osseous fishes as represent them in 

 their own circles. It seems to be a general law, that 

 tliose fishes which have the gills highly developed, and 

 the aperture very large, like the herring and mackerel, 

 very soon die on being taken from the water ; while 

 those, on the contrary, as the eel, which breathe by spi- 

 racles, live for a considerable time on being exposed 

 to atmospheric air. It is among such that we find 



