OF MOLLUSCA. 99 



1. S. planorbis, Forbes fy Hartley, B. M. t. G.G. f. 1, cop. Adams, 

 Gen. Moll. t. 35. f. 9. 



The shell of Skenea depressa is blackish when alive. Operculum 

 horny, circular. Tentacula two, filiform, white, rather short ; the 

 eyes at their external base sessile. Mouth proboscidiform. Foot 

 short, tapering posteriorly, with plain margins. The animal swims 

 with great facility in a reversed position, and in structure appears to 

 be almost identical with Rissoa. It is found on flat rocks between tide 

 marks in Berwick Bay in great profusion. — Johnston, Proc. Berw. 

 N. Club, i. 273. 



Dr. Loven also observes that the animal of Skenea depressa is 

 like Rissoa ; the operculigerous lobe with a rounded wing on each 

 side, produced and lanceolate behind ; and Mr. Clark considers it a 

 depressed species of that genus. 



Jaws (or prehensile collar) of Skenea planorbis tubercular, distinct. 

 Teeth are 3 • 1 • 3, in about 20 or 30 rows ; central four-sided, rather 

 broader than long ; apex rounded, recurved, three-toothed, and with 

 a tooth on each in front of the base ; lateral subulate, compressed, 

 rather broad at the base, slender and curved at the tvp.— Alder. 



Mantle of Skenea planorbis simple. Muzzle produced, vertically 

 cloven. Jaws horny ; tongue spinose. Tentacles short, flattish, 

 not setose. Eyes large, black, external, basal. Foot long, slightly 

 auricled. Operculigerous lobe much expanded, dilated into a sub- 

 rotund form behind, like the sole, with a very obsolete beard. Oper- 

 culum suborbicular, of many rapidly increasing whorls. — Clark, 373. 



Aperture of the shell of Skenea depressa is subcontinuous, circu- 

 lar. Operculum transparent, horny, pale yellow, orbicular, of a few 

 rapidly enlarging whorls. Animal : Tentacles two, subcylindrical, 

 filiform, blunt, transparent. Eyes distinct, a little above the outer 

 base of the tentacles. Trunk short, protruded, apex nicked at the 

 tip. Foot oblong, continuous, rather narrow, slightly truncated in 

 front, and rather tapering behind. Opercular mantle nearly twice 

 as wide as the foot, making a white fringe to the operculum, without 

 any processes. Foot folded across when contracted : the places of 

 the fold look sometimes as if the foot was interrupted there, and 

 there is a central longitudinal transparent line in the hinder part of 

 the fold. 



Fam. XIV. CMCIDM. 



Shell subcylindrical, arched ; apex subspiral, deciduous, the dorsal 

 surface convex. Operculum horny, circular, spiral. Proboscis 

 thick, truncate. Tentacles subulate, lateral. Eyes basal, exterior. 

 Foot short, truncate in front, acute behind. Gills laminar ? Jaws 

 distinct. Teeth 3 • 1 • 3 ; lateral subulate, curved. 



The branchial leaflet of Ccecum and Brochina is very minute, ap- 

 pearing like two, one larger than the other. Foot regular in shape, 

 but singularly short both before and behind. 



h 2 



