CONCLUDING REMARKS. 



369 



favourably situated, affords a considerable number of 

 species, among the most remarkable of which are Lit- 

 torina tenebrosa and Rissoa ulvse. 



Of the Cuttle-fish tribe only five species have as yet 

 occurred; but the Foraminifera have presented them- 

 selves in considerable number, our list being more ex- 

 tended than that of any other district in Scotland, although 

 species, no doubt, remain to be added, and other districts 

 are probably at least as prolific. The marine Gaster- 

 opoda are comparatively numerous ; and among them 

 have occurred several species supposed to be new. Of 

 the Nudibranchiate series, of which only twelve have 

 occurred, there must be at least three times as many in 

 our seas. Nothing general, I think, can be said respect- 

 ing the Tropiopoda, of which more than a hundred 

 species have been met with, besides the few which are 

 fluviatile. Several unexpected species have occurred, 

 and at least one that might have been looked for, the 

 Common Oyster, does not appear to exist anywhere 

 along our coasts, although shells of it are found on the 

 beaches near Peterhead, where, as Mr. Gray informs me, 

 it was introduced some years ago, but where no live in- 

 dividual has been met with of late. The Ascidise I have 

 not sufficiently searched for. They do not seem to be 

 very numerous about Aberdeen at least. 



It may be useful to say a few words to collectors of 

 MoUusca. As to shells, the best specimens are to be 

 selected, including all the varieties. The animal is to be 

 killed with hot water, and carefully removed. But fre- 

 quently univalve shells crack under this treatment ; and 

 therefore tie heat should be gradually applied to them. 

 Substances adhering to shells should be removed with a 

 penknife, but so as not to injure the surface, or with a 

 hard brush and water. The valves of the Tropiopoda 

 are to be kept closed by a thread wound round them 

 until dry. Most shells are improved in their appearance, 

 and their epidermis is preserved, by the application of a 

 very little oil to them either with a bit of soft leather or 

 a brush. 



