436 ZOOLOG T. 



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 G-anoids, 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 G-anoids, 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 P olypterus and Amia 

 the spinal column is completely bony, the vertebrse being 

 ampliiccelous, i. e., biconcave ; while in the garpike {Lepidos- 

 teiis) the vertebra 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 Elasmobranchs, the optic nerves 

 uniting 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- 

 philiian 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. 



