430 Rev. M. J. Berkeley on Rissoa subumbilicata. 
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 Helicina, which have the mantle open behind, to the 
Pectinifera. Its nearest ally however amongst the pectiniferous Mol- 
 lasca I should conceive not to be Rissoa. That an opportunity of com- 
paring the animals may be afforded, it may not be improper to subjoin 
a description of the Pissoa subumbilicata made at the same time. This 
species I have always found in brackish water amongst Conferve, Rup- 
pia, Zannichellia, &c. 
RIsSOA SUBUMBILICATA. 
Foot truncate in front, not grooved along the anterior margin, oblong, 
obtuse, the sides hollowed out ; not composed of two distinct lamine. 
Tentacula long, filiform, eyes on tubercles at their external base. 
Muzzle proboscidiform, smooth, lip scarcely any. 
Operculum corneous, spirally striate. 
Feces elliptic. 
Placed in water it swims on the surface by means of its foot. 
There are two peculiarities in this species not general in 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 Figures. 
Prats XIX. 400 fA. 297 
Fig, 3. Voluta 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 
branchiz. 
4. Assiminia Grayana. 
a, a. 
b, b. 
c. Is intended to show how the lower lamina of the foot pro- 
jects beyond the upper. 
5. Rissoa subumbilicata. 
' As in the last. 
