Mr. W. Clark on the Aclis unica. 123 



discovery that now dawns on us will dissipate, as it does in every 

 case, misapprehensions, and tell ns that the Fates have decreed, 

 we all have been at fault about a very simple creature, which 

 though not absolutely a typical Rissoa, is all but one, as the 

 shell only wants the callus on the outer lip ; but we have many 

 admitted Rissoce without that appendage ; indeed, if we were to 

 look for strict typical specialties in either the hard or soft parts 

 of any mollusk, every species must become a genus. 



Rissoa unica, nobis. 



Aclis unica, Brit. Moll. vol. iii. p. 222. pi. 90. f. -4, 5. 

 Chemnitzia unica, Alder et nobis. 

 Turritella unica, Fleming. 

 Turbo unicus, Mont, et auct. 



Shell. — Of eight yellowish-white, rounded, finely reticulated 

 volutions with oblique well-marked sutural lines. The apex is 

 obtuse and not reflexed, as stated by me in another place : I was 

 deceived by imperfect specimens, which led me into the error of 

 supposing that it would prove a Chemnitzia. 



This is one of the slenderest British shells, having only an 

 axial admeasurement of —, and a diameter of ^ unciae ; the 

 outer lip is thin, and the aperture is oblong-oval and almost 

 entire. 



Animal. — The general colour is hyaline-white, shot through- 

 out all its organs with a mixture of very minute close-set points, 

 short lines or blotches, of flaky and frosted snow colour. Mantle 

 even with the shell, except that at the apertural upper angle it 

 emits the filament I have so often mentioned as being present 

 in all the Rissoce, and whose particular function is doubtful. 

 The muzzle is slender and rather long, having the first half from 

 the neck, on its upper part, clothed with a very close tunic or 

 tight overlay ; the disk is smooth, compressed, bevelled to a fine 

 edge, and almost circular, with a median vertical fissure on the 

 under surface, in which I have often seen the delicate white cor- 

 neous plates, jaws, and lingual riband : but great powers and 

 much time are required to seize a favourable opportunity of vision 

 in so minute and restless a being. The tentacula are very like 

 those of Rissoa striata, moderately long, flat, rounded or obtuse 

 at the tips, quite smooth even under high powers, divergent, 

 with large black eyes, not on pedicles or prominences, but fixed 

 on the centre of their bases with very little external inclination, 

 and widely apart ; there is no connecting tentacular veil, nor the 

 least triangularity, foldings, or the presence of apical inflations, 

 as in the Chemnitzia ; on the march the eyes are usually carried 

 within the margin of the shell. The foot is slender, greatly hol- 

 lowed out in front and deeply labiated, with distinct, long, 



