AND ON THE CLASSIFICATION OF FISH. 517 
even in the cutaneous covering. In the true Sturgeons the 
large osseous plates are placed in longitudinal rows, which are 
separated by a considerable interval in Scaphirhynchus; the 
posterior part of the body is uniformly covered with ganoid 
plates. But the ordinary Sturgeons likewise possess true ganoid 
plates at the sides of the tail. Moreover, the fulcra of the sum- 
mit of the elongated upper tail-piece are as in Paleoniscus, Acro- 
lepis, &c. No one who saw the tail only of a sturgeon would 
hesitate to pronounce it to be the tail of a heterocercous Ganoid, 
Taking all things into consideration, the only true Ganoids 
of the present time are the genera Polypterus, Lepisosteus, Act- 
penser, Scaphirhynchus, and Spatularia. This result, independ- 
ently of its immediate interest, is also remarkable, because it 
relates to the fish with which Cuvier compared Paleoniscus in 
1824, It was not this great philosopher’s intention to place the 
Sturgeons, Polypterus and Lepisosteus, in one section with the 
Paleoniscus of the Zechstein ; it may rather be proved that this 
idea was entirely foreign to his mind. In 1828, in the new 
edition of his ‘ Animal Kingdom,’ he has still arranged the Stur- 
geons in his section of cartilaginous fishes, Lepisosteus and 
Polypterus among his osseous fishes, in the abdominal Malaco- 
pterygian family of the Clupee. His idea, which he expressed in 
definite words, was simply that the Paleoniscus should either be 
united with the Lepisosteus and Polypterus, or with the Stur- 
geons; that the determination of this depended upon certain 
doubtful points ; and he inclined to the opinion, which was more 
distinctly expressed by Valenciennes, that Palgoniscus and Di- 
pterus belonged with Lepisosteus to the abdominal Malacoptery- 
gians. 
The characters of the Ganoids when summed up are the fol- 
lowing:—They are fishes provided with either tabular and angular 
or round enamelled scales, or with bony plates, or a perfectly 
naked skin. Their fins are frequently, but not always, covered 
on their anterior border with a single or double row of spiny 
plates or laminz. Their caudal fin sometimes receives in its 
upper fold the extremity of the vertebral column, which may 
continue as far as the extremity of that fold; their double nasal 
apertures resemble those of the osseous fishes; their gills are 
free and lie in a gill-cavity covered by an operculum, as in the 
osseous fishes, Several have an accessory respiratory organ 
in the form of an opercular gill, which must be distinguished 
: 2P2 
