668 ' . PHILOSOPHICAL TRANSACTIONS. [aNNO 1773. 



of the animal. It becomes thinner again, towards the lower surface or belly ; 

 but that edge is not so thin as the other. Its union with the parts to which it is 

 attached is in general by a loose, but pretty strong, cellular membrane ; except 

 at the partition, to which it is joined so close as to be almost inseparable. 



The small organ lies along the lower edge of the animal, nearly to the same 

 extent as the other. Its situation is marked externally by the muscles which 

 move the fin under which it lies. Its anterior end begins nearly in the same 

 line with the large organ, and just where the fin begins. It terminates almost 

 insensibly near the end of the tail, where the large organ also terminates. It is 

 of a triangular figure, adapting itself to the part in which it lies. Its anterior 

 end is the narrowest part ; towards the tail it becomes broader ; in the middle of 

 the organ it is thickest ; and from thence becomes gradually thinner to the tail, 

 where it is very thin. The two small organs are separated from one another by 

 the middle muscle, and by the bones on which the bones of the fins are articu- 

 lated. The large and the small organ on each side, are separated from one 

 another by a membrane, the inner edge of which is attached to the middle par- 

 tition, and its outer edge is lost on the skin of the animal. To expose the 

 large organ to view, nothing more is necesary than to remove the skin, which 

 adheres to it by a loose cellular membrane. But to expose the small organ, it is 

 necessary to remove the long row of small muscles which move the fin. 



Of the structure of these Organs. — The structure is extremely simple and 

 regular, consisting of 2 parts ; viz. flat partitions or septa, and cross divisions 

 between them. The outer edge of these septa appear externally in parallel lines 

 nearly in the direction of the longitudinal axis of the body. These septa are 

 thin membranes, placed nearly parallel to one another. Their lengths are nearly 

 in the direction of the long axis, and their breadth is nearly the semidiameter 

 of the body of the animal. They are of different lengths, some being as long 

 as the whole organ. I shall describe them as beginning principally at the 

 anterior end of the organ, though a few begin along the upper edge ; and the 

 whole, passing towards the tail, gradually terminate on the lower surface of the 

 organ ; the lowermost at their origin terminating soonest. Their breadths differ 

 in different parts of the organ. They are in general broadest near the anterior 

 end, answering to the thickest part of the organ, and become gradually narrower 

 towards the tail, however they are very narrow at their beginnings or anterior 

 ends. Those nearest the muscles of the back are the broadest, owing to their 

 curved or oblique situation on these muscles, and get gradually narrower 

 towards the lower part, which is in a great measure owing to their becoming 

 more transverse, and also to the organ becoming thinner at that place. They 

 have an outer and an inner edge. The outer is attached to the skin of the 

 animal, to the lateral muscles of the fin, and to the membrane which divides 



