122 CLASSIFICATION OF FISHES. 



shall now endeavour to make out, and subsequently 

 demonstrate. 



(lOS.) Of all the cartilaginous fishes yet discovered, 

 that which seems to make the nearest approach to the 

 osseous orders, is the Polyodoii reticulatus {Jig. 7.), a 



most extraordinary fish, about a foot long, found in 

 the Mississippi. It is at once known by the excessive 

 prolongation of the snout, which is very flat and lan- 

 ceolate, or broadest in the middle, while its length is 

 nearly equal to that of the whole body. The skin is 

 smooth and destitute of scales. The general structure 

 shows an affinity to the sturgeon, close to which it has 

 always been arranged ; but it differs from that genus in 

 some important particulars, besides presenting a totally 

 different form. The maxillary and palatal bones, indeed, 

 are united ,• but the pedicle of the mouth has two 

 articulations. The mouth itself is wide, and is furnished 

 in the upper jaw with a double, and in the lower with 

 a single, row of small, but sharp, curved and serrated 

 teeth. In all these respects, however, we still have the 

 general characters of a cartilaginous fish ; but by its 

 other characters we trace its connection to those whose 

 bones are osseous. The spiracle, common to the rest 

 of this order, is so large as to assume the appearance of 

 the branchial aperture of ordinary fishes ; for both 

 Lacepede and Cuvier affirm that it extends to the 

 middle of the body. It is covered by a very large, 

 soft, and pointed operculum, which, on being raised, 

 exhibits the gills, consisting of five cartilaginous lamina, 

 with fringed edges, as in the generality of fishes. Like 

 Acipenser, there is a large swimming bladder : the in- 

 testine is provided with the spiral valve common to this 

 order ; but the pancreas, according to Cuvier, exhibits 



