AND ON THE CLASSIFICATION OF FISH. 521 
naked, so do some such undoubtedly occur among those which 
are fossil; these could not be recognised from the structure of 
the surface of the body, but from their affinities to other genera 
only, and partly from the condition of the vertebral column. 
The cartilaginous structure of the central portion of the ver- 
tebral column alone will not positively decide whether a fish fur- 
nished with scales be a Ganoid; for we have in Lepidosiren an 
example of a group of scaly fish distinct from the Ganoids, pro- 
vided with a cartilaginous centre to the vertebral column. 
I now come to the systematic arrangement of the Ganoids. 
In the first place, we must admit that they form one of the 
largest divisions of fish, whether they be considered as an order 
or subclass, and that they do not merely constitute a single 
family. As long as the peculiar differences in the anatomy of 
the Ganoids from that of the osseous fish, as in the structure 
of the valves, the optic nerves, the respiratory organs and the 
organs of generation, were unknown, the position of those fish 
which agreed with Lepisosteus and Polypterus in the structure 
of the scales was doubtful, whether they constituted an order or 
a family of the Malacopterygii Abdominales, For if the Lopho- 
branchii, Gymnodontes and Sclerodermi were separated from 
the Ganoids, the latter agree with the Malacopterygii Abdomi- 
nales in the possession of the air-duct of the swimming-bladder, 
the position of the ventral fins, and the soft structure of the fin- 
rays. For this reason, in my treatise on the natural families of 
the osseous fish, I left Lepisosteus and Polypterus in the order in 
which Cuvier had placed them, i. e. among the abdominal Ma- 
lacopterygians, but as a separate family. In the present state 
of my knowledge this is inadmissible. It is evidently proved 
that these fish fundamentally differ from the osseous fishes. 
They have equally little claim to be arranged among the Sela- 
chit ; at the same time, as they approximate to one portion of 
the previous cartilaginous fishes, they form a separate section. 
The systematic position of this section, as I think I have proved, 
lies intermediate between the osseous fishes and Plagiostomi 
or Selachii, combining the characters of the osseous fishes and 
the Selachti. It has the gills, the opercula and the snout of 
the former, and the accessory gill to the first gill, the blowing- 
hole, the valves, the vascular subdivision of the pseudo-branchia, 
the oviduct and the structure of the optic nerve of the latter. 
It may be stated that some animals in this section approximate 
