KEY-HOLE LIMPETS. 215 



In the genus Siphonaria the same organ is within a 

 cavity similarly placed, and the canal or siphon is formed 

 by the passage of water for purposes of respiration. 



The Parmophorus has a communication between the 

 branchiee and the water, from the anterior side of the head ; 

 the shell is nearly flat, and merely indented at the edge 

 with a kind of sinus. 



The breathing organ in this genus Emarfjimila varies 

 between that of the Farmojjhori and Fissurell(E ; the shell 

 is small, shaped either like a depressed or slightly elevated 

 Cone. 



In the genus Fissurella the shells are perforated at the 

 summit, for the purpose of conveying water to the respira- 

 tory cavity. This perforation, which is generally of an 

 oblong oval shape, was especially noticed by early natu- 

 ralists ; and although the FissurellcB, or Key-hole Limpets, 

 were usually regarded as distinct from the PatellcBy or 

 common Limpets, they were not elevated to the rank of a 

 genus. Bruguiere was the first to accomplish this desirable 

 separation, and the genus has now become one of consider- 

 able interest on account of the new and beautiful species 

 that have been contributed by Mr. Cuming, and which, 

 mostly exhibiting an elegant variety of painting, are highly 

 esteemed by collectors. 



