Mr. W. Clark on iVssimiuia Grayana and Rissoa auatina. 117 



ously labiated; when not in action rounded posteally, but in 

 full progression it assumes a broad lancet-shape, suddenly sloping 

 on each side to an obtuse termination ; there is no longitudinal 

 depressed line on the sole, and only a slight transverse crease is 

 visible when the posterior portion is drawn up to the advanced 

 one ; it carries on a distinct upper fuscous lobe, which is only 

 narrowly alated on each side, a light corneous pear-shaped 

 operculum, with a rather indistinct submarginal nucleus, the 

 spiral continuations of w^hich show the fine oblique lines of in- 

 crement, as in the Littorince and RissofP. The animal is slow 

 and deliberate in progression ; its action is between that of Lit- 

 torina and Rissoa, not having the lateral oscillatory march of 

 the former, nor the perfectly steady advance of the latter. The 

 foot on the upper surface is of all the hues of lead-colour ; the 

 sole is pale bluish-white, aspersed thickly with minute flakes of 

 an intenser white. 



From the non-transparency of the shell, the character of the 

 branchiae could not be well made out, and when examined after 

 extraction theii- delicate and minute structure sufl"ered from col- 

 lapse. The neck has no appendages except the guide lines for 

 the branchial w^ater, and the muzzle is altogether without them. 

 Verge long, slender, and falcate at the end. 



It inhabits the Greenwich marshes in company with the Rissoa 

 anatina, and seems, like many of the littoral species, to enjoy a 

 kind of amphibious existence ; it is also nearly equally at home 

 in fresh water, or in a mixture of salt, half of each. It feeds on 

 a common Ulva of the jdooIs. I received the animals by post, 

 deposited in a small tin box in moderately moistened weed ; they 

 remained alive for some time, but they are constitutionally 

 sluggish. 



The above minutes of this curious creature do not differ in 

 any essential character from my recorded descriptions of the 

 TruncatellcB, and will justify the surmises I have expressed in the 

 earlier part of this account, that Assiminia is only a generic 

 s\Tionym of the established genus Truncatellu. 



Rissoa anatina, auctorum. 

 Rissoa anatina, Brit. Moll. vol. iii. p. 134. pi. 87. f. 3, 4. 



Shell — of a light horn-colour, of 4-5 rounded tumid volutions, 

 with well-marked wrinkled striee of increment, as in Bithynia 

 ventricosa {Leachii, nonnulL), to which, in these respects, it bears 

 much resemblance, as well as in the entire peristome ; the two 

 are also occasionally found in the same locality, with the colour 

 of their shells obscured by a similar black earthy deposit. This 

 circumstance is singular, as the Truncatella Grayana living with 



