4 Mr. W. Clark on the Genus Tnincatella. 



viable. In specimens of C. plumaris from the West Indies com- 

 municated by Agardh and Mertens, the piunse are very nearly 

 twice as long as in other specimens from the East Indies and the 

 Cape of Good Hope ; and we must not forget that their extreme 

 length (nearly 1 inch) forms the only specific difference of Cau- 

 lerpa longifolia, an Australian species. With regard to the pinnae 

 of these perplexing forms I may further add, that, in not being 

 attenuated at the base, they are completely separated from Cau- 

 lerpa taxifulia. 



Plate II. fig. I. C.fissidentoides, natural size. Fig. 2. A portion of the 

 frond magnified. 



II. — On the Genus Truncatella. By William Clark, Esq. 

 To the Editors of the Annals of Natural History. 



Gentlemen, Exmouth, June 8, 1853. 



Mr. Wm. Thompson of Weymouth has this day favoured me 

 with some lively examples of the rare Truncatella Montagui in 

 its adult and young states, that is, before and after the truncature 

 of the apex, and also others of the still rarer Rissoa littorea of 

 authors ; these, though sent by post in moistened weed, which 

 however was quite dry when received, on being put into sea water 

 immediately deployed the organs. The first has been described 

 by the Rev. R. T. Lowe, many years ago, in the 5th volume of 

 the ' Zoological Journal,' p. 303 ; and the R. littorea by the 

 learned authors of the 'British Mollusca' in the Appendix, 

 vol. iv. p. 265 ; still, as my account contains many new particulars, 

 and notes a difference of opinion in respect of the generic posi- 

 tion of R. littorea, I have thought it would be agreeable to some 

 of your readers to have an accessorial description of these rare 

 creatures from a fresh though inferior hand, especially as the 

 present notes exhibit a comparative view of both animals, which 

 were examined for two days in the same vase under very favour- 

 able circumstances. It is singular that these species, both un- 

 doubted Pectinibranchiata, should inhabit at high water level, 

 in company with Conovulus denticulatus and C. bidejitatus, both 

 Pulmonifera, and are, as Mr. Thompson has informed me, " deci- 

 dedly amphibious," being often found far above that limit ; but 

 I apprehend not more so than many of the minuter Littorince, 

 particularly L. nerituidcs, which are attached to rocks for long 

 periods, perhaps during their whole existence, above the highest 

 tides and even beyond the reach of the spray, living apparently on 

 the tloatiug saline moisture. It would aj)pe;ir then that the two 



