430 Rev. M. J. Berkeley o)i liissoa suhumhilkuta. 



sequently made a distinct genus for its reception. In many points the 

 animal resembles very much that of Cyclustoma, and is perhaps a step 

 nearer than that and Ilelicina, which have the mantle open behind, to the 

 Pectinifera. Its nearest ally how^ever amongst the pectiniferous Mol- 

 /wsca I should conceive not to be /?zs«oa. That an opportunity of com- 

 paring the animals may be afforded, it may not be improper to subjoin 

 a description of the liissoa sabumbilicata made at the same time. This 

 species I have always found in brackish water amongst Conferva, Rup- 

 pia, Zannichellia, &Cc 



RiSSOA SUBUMBILICATA. 



Foot truncate in front, not grooved along the anterior margin, oblong, 

 obtuse, the sides hollowed out ; not composed of two distinct laminse. 



Tentacula long, filiform, eyes on tubercles at their external base. 



Muzzle, proboscidiform, smooth, lip scarcely any. 



Operculum corneous, spirally striate. 



Faeces elliptic. 



Placed in water it swims on the surface by means of its foot. 



There are two peculiarities in this species not general jn Rissoa, the 

 hollowing out of the side of the foot and the absence of a short filiform 

 appendage on each side of the posterior end of the foot above. 

 Reference to Fifjvres. 

 Plate XIX. 

 Fig. 3. Valuta denticulata. 



a, a, a. Shell with the animal in different positions. 



b, b. Different forms assumed by the foot. 



c. Pulmonary veins and sac of viscosity. 



d. Sac of viscosity opened to show that it is not a plume of 



branch ise. 

 4. j^ssiminia Grayana. 



a, a.i 



b,bS 



As in the last. 



c. Is intended to show how the lower lamina of the foot pro- 

 jects beyond the upper. 

 Rissoa sribumbilicata . 



