OF NEW SOUTH WALES. 243 



This definition differs in important particulars from that of 

 Philippi, Crosse, or Adams, but principally in the thickening uf 

 the throat and the oblong paucisprial operculum. No authors 

 have hitherto described the operculum in detail. 



M. Quoy in his very elaborate notice of Trochus melanostomus 

 and T. nanus, now recognised »»s Risella melanostoma and I', nana, 

 gives f 11 details of the anatomy. He states {Voy. de V Astrolabe, 

 Zool. vol. 3, p. 271-278) that the sexes are distinct. After having 

 dissected a very large number of specimens in Tasmania, I am 

 convinced that the animals are truly hermaphi*oclite, and are pro- 

 vided in every species with male and female organs. Nevertheless, 

 the sexes seem to be distinct, because some take the office of the 

 male and some the female. In such cases the shells differ, and 

 have been hitherto regarded as distinct species, whereas they are 

 only male and female shells, as I shall presently show. 



Mons. Crosse enumerates no less than nine species of the genus, 

 all of which are indigenous to Australian seas. The following is 

 his list : — 



Risella melanostoma, Chemnitz (V) Gmelin, Crosse, and Angas. 



Hub. Port Phillip, var. S. Australia. Angas. 



Risella aurata, Quoy, Deshayes (in Lamarck) Philippi, Risella 

 lutea, H. an 1 A. Adams. 



JrJab. D'Entrecasteaux Channel, Tasmania, Quoy; St. Viucent's 

 Gulf, S. A., Crosse. 



Risella nana, Lamarck, Quoy, Delessert ; Littorina australis, 

 Gray, Bembicium nanum Philippi, pictum, idem R, nana, H. and 

 A. Adams, Chnu. manuel. 



Hob. Storm Bay, Tasmania, Quoy ; " sur quelques points du 

 grand continent Australien." Crosse. 



Risella plana, Quoy, Phillippi, Adams. 



Hob. "Western Port, Quoy ; St. Vincent's Gulf, Crosse ; Port 

 Jackson, Angas. 



Risella lutea, Quoy, Kiener, Philippi : Trochus cicatricosus, 

 Jonas. 



Hab. Brackish waters, Western Australia ; Port Jackson, 

 Angas. 



