of the British Rissose. 257 



scarcely differs from the R. parva, except in having the tips of the 

 tentacula rather flatter, more rounded, and in the different posi- 

 tion of the constriction of the foot. The animal is active, marches 

 up a glass with uncommon rapidity, and displays a freedom be- 

 yond the usual habits of the tribe. It is found in all the zones. 



Some live examples of this species having occurred, I add to 

 the above account, that the front part of the foot is mai'ked with 

 an intense snow-white flake of the figure of the letter V, visible in 

 consequence of its transparency above and below ; I have also to 

 remark, that the anterior terminal line of the foot is unusually 

 deeply incised, so as to form two labia ; the lower, or that of the 

 sole, at the centre part, on the march is produced much beyond 

 the upper lip. I have never before seen this feature so exten- 

 sively developed in any Rissoa, 



And lastly, I state, that I failed to detect satisfactorily the 

 small pendent process in the mantle at the aperture, which is so 

 conspicuous in R. parva and jR. semistriata ; yet it may exist : I 

 had the same difficulty in R. striata, but afterwards I saw it in 

 several examples. 



Rissoa reticulata, Montagu. 



Rissoa Beanii, nonnull. 

 Rissoa sculpt a, nonnull. 



The animal occupies a spiral, cancellated, pale yellow or brown 

 shell of 5-6 tumid volutions; its ground colour is yellowish 

 white. Mantle even with the shell, except the filamentary pro- 

 cess at the angle of the aperture. Head proboscidiform, fur- 

 nished with the usual cloven disk and buccal appendages; the 

 rostrum near its termination at the upper surface appears to have 

 attached to it two very small similar shields, one on each side, 

 independent of the terminal minute subcircular flat lobes. The 

 tentacula are compressed, slender, rather long, rounded at the 

 extremities, not setose ; the eyes are at the external angles, on 

 short light yellow or orange pedicles. Foot subrotund, scarcely 

 auricled, but grooved in front sufficiently to form a shallow 

 labium, slightly constricted anteriorly, at one-third the length, 

 gently tapering to a rather obtuse lanceolate but not emarginate 

 termination ; the operculum is carried on an upper plain mode- 

 rately alated lobe, at nearly the point of the junction of the foot 

 with the body ; it is pyriform, with indistinct rapidly increasing 

 paucispiral gyrations; the terminal part of the lobe, like the 

 R. semistriata, is furnished with three blunt, cylindrical, short 

 cirrhi, and occasionally one of the sides of the main foot is emar- 

 ginate. The branchial plume is composed of 12-15 single, pale 

 yellow, short strands, which are visible when the neck is greatly 



Jnn. ^c Mac/. N. Hist. Ser. 2. Vol. x. ] 7 



