Mr. W. Clark on the Lacunae. 33 



lappets, and that the opercuhgerous lobe is small, simple, and 

 not co-extensive with its organ. 



Rissoa ulvce. 



Having repeated the examination of two of the varieties of 

 R. ulvcB found at Exmouth, the one having a pale horn-colour 

 shell with flat volutions, the other tumid and red-brown, with 

 the animal rather larger and of a darker lead-colour than the 

 former, I am enabled to state, that I cannot discover the 

 slightest organic difference between the two, and their action and 

 habitudes are identical ; the variation arises from habitat ; the 

 one, the pale variety, is found lurking under stones, the other 

 exposed in the open patches of the green oozes of the estuary. 

 The shells are so different in aspect, that the conchologist would 

 pronounce them distinct, but malacology steps in, and offers a 

 practical example of the superiority of its attributes by showing 

 the two to be identical. Both the varieties have the under part 

 of the foot aspersed with sulphur-yellow, opake, minute flakes. 

 The opercuhgerous lobe fully covers that organ, and extends a 

 little on each side, beyond the pedal disk, or forms what by some 

 are called minute wings. The foot is perfectly rounded behind, 

 and in almost all specimens more or less emarginate, though in 

 some scarcely visible, in others decidedly so at several points of 

 the arcuation ; it is truncate and well auricled in front, without a 

 medial line. There is invariably a very short cylindrical process 

 or fillet exserted from that part of the mantle which lines the anal 

 canal or upper angle of the aperture. I am unable to say anything 

 as to its use : whether such an appendage exists in other minute 

 congeneric species, and from its minuteness has passed without 

 observation, must be left for future examination. I consider this 

 species a Rissoa : though it has not the decided caudal filament, 

 and pointed pedal termination of the typical Rissoa, we must not 

 forget that there are some Rissoa with rounded tails and rudi- 

 mentary or obsolete posterior terminations. 



Rissoa parva. 



To my string of varieties attached as synonyms to this species, 

 I beg to add all those of the R. inconspicua ; and I believe the 

 R. labiosa of authors is only an elongated variety of it, as the 

 former is a dwarf one, the "parva" of the coralline zone; the 

 other is the "parva " of the highest parts of the littoral zone. We 

 thus see the one is rendered dwarf by the absence of light, green 

 food, and depth of water ; the other having all these advantages, 

 shows their effects in a more exuberant animality. 



I beg that the R. scalariformis attached as a synonym to R, 

 Ann. ^f Map. N. Hist. Ser. 2. Vol. vi. 3 



