118 Mr. W. Clark on Assiminia Grayana and Rissoa anatina. 



or near them is perfectly clean : perhaps the other two have 

 some difference of habitat. Axis /^^^^^ diameter g^ths of an 

 inch. 



Animal. — The mantle is simple, and the linear process seen at 

 that part which lines the upper angle of the aperture in Rissoa 

 ulva, and often in other Rissoa', is wanting. The muzzle is long, 

 subcylindrical, and armed with very contractile, dark, annulated 

 ridges, which allow of a great protrusion of the neck. The 

 buccal orifice is cloven vertically, and from it, almost without 

 cessation, the animal shows a pair of corneous jaws, to which the 

 short lingual riband proceeding from the stomach is united, and 

 both portions of the tongue are supported by a pair of red fleshy 

 plates, which are visible through the tenuity of the enveloping 

 tissue. 



The tentacula are long, not much flattened, slender and mode- 

 rately pointed, of a very light ashy hue, quite smooth, divergent, 

 with rather large black eyes at the external bases, fixed on mi- 

 nute semicircular expansions ; at tlieir lower half they are spa- 

 ringly studded with white, flaky, minute irregular blotches, and 

 sometimes a very fine cinereous line coasts their margins. 



The foot is scarcely so proportionately long, slender, and con- 

 stricted below the auricles as in the usual run of the Rissoa ; it 

 is perfectly rounded postcally, but parts of the margin are some- 

 times notched; anteally furnished with moderately large lateral 

 auricular cx})ausions. The opercular lobe is distinct, and shows 

 at the junction of the foot with the body a whitish alatcd process 

 on each side, and carries a rissoidean suboval operculum of laxly 

 spiral rather indistinct turns, with an eccentric nucleus; the 

 curved lines of each volution are coarser, fewer, and less oblique 

 than in the generality of the Littorince and Rissoa. The upper 

 part of the main foot is of a darkish lead-colour, disposed in 

 close-set fine irregular lines ; the sole is either white, yellow or 

 palely cinereous, without a longitudinal depressed central line, 

 and aspersed thickly with minute pale-gray-colourcd points or 

 flakes. The opercular lobe is without a caudal process, but it 

 may be observcid, tliis is not an invariable adjiuict of all Rissoa. 



The clmracter of the branchial plume in a young, almost 

 white horn-coloured shell was suffieii'utly apparent ; it consists 

 of a flat or(linary-slia])ed leaf deposited in the usual position in 

 the branchial vault, of 12 to 16 or even more coarse white strands ; 

 the transverse measui'c of the anterior ones and those nearest 

 to the pc'ricai'dium being the shortest, whilst the central threads 

 are gradually elongated. The intestine is rendered visible, by its 

 contents, tlirougli tlie filmy shell, — at least that length of it which 

 |)r()C(;eds from the part of the animal enclosed in the second 

 volution to its debouchuie as rectum on tlie right side. Verge? 



