Chap. III.] 



G A NO I DEI. 



9 1 



the gill chamber is 

 covered in by a 

 bony plate, oper- 

 < n lima ; the renal 

 ducts do not open 

 into a depression 

 (cloaca,) common 

 to them and the 

 anus. In all Ga- 

 noids, and in one 

 great division of 

 the Teleostei, the 

 air sac on the dor- 

 sal surface of the 

 body opens by a 

 duct into the oeso- 

 phagus. 



The recent Ga- 

 noidei fall into two 

 divisions : 



a. Selachoi- 

 dei ; * such are 

 the Sturgeons (Aci- 

 penser) and Poly- 

 odon ; in these the 

 skull consists of 

 persistent cartilage, 

 overlaid by bones 

 developed in the 

 investing mem- 

 brane ; spiracles 

 are persistent, and 

 the body is either 

 naked, or has bony 

 plates developed in 

 the dermis (Fig. 46). 



* Chondrostei. 



