262 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



the fish heart. This bulbiis arteriosus is very different from 

 the bulbus aortce of other fishes, where it is simply a swelling 

 of the walls of the aorta, not contractile, without valves in 

 the interior, and separated from the heart by two valves op- 

 posite to each other. This character is also supported by 

 two others of great importance, viz., the presence of a spiral 

 valve in the intestine, which is found in a more or less de- 

 veloped state in all the ganoids, but entirely absent in other 

 fishes ; and by the optic nerves being placed side by side, and 

 not decussating as in ordinary fishes. The occurrence of the 

 chimeras as an intermediate rank between the plagiostomes 

 and ganoids is considered as strengthening the view thus ta- 

 ken, and accordingly Dr. Gtinther proposes the name Palce- 

 ichthyes for this sub-class ; the remaining orders of fishes be- 

 ing distinguished, as already indicated, by possessing a two- 

 chambered heart with a rigid bulbus aorta?, and decussating 

 optic nerves, and in never exhibiting a trace of spiral valve 

 in the intestine. 



Of the pew sub-class indicated by Dr. Gtinther there are 

 now known 140 species of sharks, belonging to 34 genera, and 

 150 species of rays, of 25 genera, and inhabiting all the seas 

 of the globe, but decreasing in numbers from the tropics to- 

 ward the poles, very few of them entering, or at least living 

 in fresh water. These constitute the order Plagiostomata. 

 In the order Holocephala there are but four species, viz., three 

 (Jhimmras and one Callocephalus, these being restricted to 

 the seas of the temperate zones of both hemispheres, and ab- 

 sent between the tropics. 



The order of Ganoidei is composed of fresh-water species ; 

 one of Amia from North America; three of Lepidosteus / 

 two of Polypterus from Africa ; two of Polyodon, or shovel- 

 nosed sturgeon, one of them found in the Mississippi and the 

 other in China ; about twenty-five sturgeons from the north- 

 ern hemisphere ; two species of Ceratodus from tropical Aus- 

 tralia ; one of Lepidosiren from the Amazon River ; and one 

 of ProtopUras from tropical Africa. 



As the total number of fishes known at present is about 

 9000, the sub-class of Palceichthyes forms only 3.6 per cent, 

 of the number. Dr. Gtinther is, however, of the opinion, 

 from the extent of the regions hitherto unexplored in respect 

 to their fishes, that perhaps we are scarcely acquainted with 



