124 Mr. W^ Clark on the Aclis unica. 



arcuated linear auricles which play on the march, or, as M. Loven 

 would term it, " late vibr antes" beneath which it is slightly con- 

 stricted, and a little beyond the middle, posteally, is fixed on a 

 simple lobe without lateral expansions or terminal cirrhal fila- 

 ment ; the light yellow suboval operculum with distinct grossly 

 spiral turns, exactly as in the paucispiral Littorinidce ; below the 

 operculum the foot is visibly contracted on each side, and ter- 

 minates in a rounded rather broad point; no median line is 

 apparent in any part of the sole. 



This creature is not at all shy ; it remained lively for thirty-six 

 hours, and gave every facility for good examination ; it readily 

 creeps up the deepest glasses, and however often brushed down, 

 starts again with unabated vigour. The specimen described was 

 detected in Littleham Cove, between Exmouth and Budleigh 

 Salterton, in the littoral level, in a debris of minute decayed 

 shells mixed with sand and mud that has an ofi"ensive odour, the 

 mass being deposited on the margins of deep quiet pools afford- 

 ing nutriment to certain long narrow grassy sea-weeds. I have 

 been thus particular to obviate difficulty to future naturalists, 

 and I wish them success in obtaining a live specimen with less 

 trouble than I have had. 



The habitat of this species is, I believe, strictly littoral ; its 

 associates are the Rissoa parva, R. striata, R. planorbis, nobis 

 {Skenea planorhisy auct.), as these are found in the same mass of 

 spoil. 



That this is a Littorinidan and almost a strict Rissoidean 

 animal, allowing a trifling margin for specialty- variations, ad- 

 mits of no doubt. It has no malacological community with Tur- 

 ritella, Aclis, or Chemnitzia j but as the muzzle is carried in nearly 

 a similar position as in the latter genus, the young malacologist 

 must take care in so small an object not to be misled by this 

 circumstance, or by the centrality of the eyes at the base of the 

 tentacula : but the veteran observer with delicate and apt mani- 

 pulation, patience, and good glasses, will easily detect the verti- 

 cally cloven disk and corneous jaws, which, with the rissoid 

 simple tentacula, will demonstrate that this animal is merely an 

 elongated Rissoa ; and in our volume of malacological observa- 

 tions now in the press, we shall deposit it in the section of the 

 elongated species of that genus. 'j-.m;^ 



I am. Gentlemen, 

 Your most obedient servant, 



William Clark. 



P.S. — The remarks of Dr. T. Williams on my branchial theory, 

 when finished, will be duly acknowledged either in the ^Annals,' 

 or in the appendix to my " Malacological Observations" now in 

 the press. 



