426 ZOOLOGY. 



that there is a natural series of forms leading from the stur- 

 geon, which is nearest the Elasmobranchs, up through the 

 Dipnoans to the true Ganoids, and that the latter, through 

 Amia, leads to the bony fishes, we shall have a clue to the 

 intricate relations existing between them and the other sub- 

 classes of fishes.* 



The Ganoids of the present day are well nigh confined to 

 fresh water, the sturgeons alone living in the sea as well as 

 ascending rivers ; though the Devonian and carboniferous 

 forms occur as marine fossils. 



In synthetic forms, like the Ganoids, it is difficult to find 

 absolute characters separating them from the Elasmobranchs 

 on the one hand and the Teleosts on the other. The diag- 

 nostic characters are the following : the skeleton is either 

 wholly cartilaginous, or partly or wholly bony ; the skin is 

 either smooth, or with cycloid, or usually with ganoid scales ; 

 the gills are free ; the gill-opening is covered with an oper- 

 cular bone ; the first fin-rays generally sharp ; the air-blad- 

 der with a pneumatic duct ; the embryos sometimes with ex- 

 ternal gills. 



The spinal column is usually cartilaginous ; in the Dip- 

 noans, the sturgeons, the paddle-fish and allies, the notochord, 

 with its sheath, is persistent ; while in Polypterus and Amia 

 the spinal column is completely bony, the vertebrae being 

 amphicoslous, i. e., biconcave ; while in the garpike (Lepidos- 

 teus) the vertebras are convex in front and concave behind. 

 The cartilaginous skull is covered by broad, thin membrane- 

 bones, as seen in the sturgeon. The tail is heterocercal, the 

 lobes being, in Amia, nearly equal. 



The brain is as in the bony fishes, but the optic nerves 

 unite in a chiasma. The heart and aortic bulb are as in the 

 Elasmobranchs, and all but Lepidosteus have a well-devel- 

 oped spiral valve in the intestine, the valve being rudimentary 



* Although strongly inclined to regard the Dipnoans from their am- 

 phibian and reptilian characters as types of a subclass, Dipnoi, yet in 

 deference to the principles stated by Gill, which we had previously fol- 

 lowed independently in the classification of the neocaridan and palseo- 

 caridan Crustacea, we here adopt the classification of Prof. Gill. 



