38 BOARD OF AGRICULTURE. 



They vary in number, situation and structure in the different genera 

 and species. The size of the fins is also equally various in the 

 different species, as it bears no constant proportion to the figure 

 or magnitude of the fish, nor to its habits or instincts. 



The situation of the fins furnishes to the Ichthyologist some of the 

 most obvious and useful distinctive characters, and have there- 

 fore received names expressive of their respective locations. 



Ventrals placed forward of the pectorals he termed Jugulars. Those 

 that had the ventral fins beneath the pectorals he called Thoracic. 

 Those that had the ventral fins placed behind the pectorals were 

 called Abdominal. This system has now given way to one more 

 in accordance with natural orders ; but these distinctions are of 

 great use in marking characteristic differences between genera 

 and species. 



In regard to the structure and operation of the fins of fishes, we 

 make an abstract of remarks by a writer in the Ed. Encyclopedia 

 on this subject. In general these organs consist of numerous 

 pointed rays, which are sub-divided at their extremities. These 

 are covered on each side by the common integuments, which 

 form in some instances soft fibres projecting beyond the rays. 

 These fins, with articulated rays, were considered by the older 

 Ichthyologists as furnishing characters for systematic arrange- 

 ment of great importance. Fishes possessing these were termed 

 Malacopterygii. 



Besides these articulated rays, there exists in the fins of some fishes 

 one or more rays made up of a single bony piece, enveloped like 

 the former by a common membrane. Some fishes have one or 

 more fins consisting entirely of these bony rays. Fishes with 

 such rays are called Acanthoplerygii. In a few genera the pos- 

 terior dorsal fin is entirely destitute of rays and has obtained the 

 name oijinna adiposa, or adipose fin, as in the salmon and trout. 



As these rays serve to support the fins, and are capable of ap- 

 proaching or separating like the sticks of a fan, we may conclude 

 that they move upon some more solid body as a fulcrum. Ac- 

 cordingly we find in the sharks, for example, that the rays of the 

 pectoral fins are connected by a cartilage to the spine. In the 

 osseous fishes the pectoral fins are attached to an osseous girdle, 

 which surrounds the body behind the branchice (gills), and which 

 supports the posterior edge of their apertures. This osseous 

 girdle is formed of one bone from each side, articulated at the 



