— -— >. 
25 
coarsely marked with concentric wrinkles ; nucleus, posterior and 
lateral, forming a small cavity at that portion of the shell. 
Length 5 lines. 
18. PLEUROBRANCHUS MARGINATUS. 
Form oval, smooth, convex above and subpellucid. Mantle widest 
at the middle, rounded behind and truncately rounded in front, and 
concealing the foot. Tentacles rather long, stout, grooved, truncated, 
and cylindrical. Oral veil triangular. Foot oblong oval. Colour pale 
lemon-yellow, freckled with white and margined with light red. Shell 
ovate, thin, fragile, pellucid, whitish horn-colour, with a dull red 
tinge near the nucleus. Nucleus subspiral. Strie of growth coarse. 
Under stones in the lower region of the littoral zone. 
19. PLEUROBRANCHUS RUFUS. 
Form oval, smooth, and convex above. Mantle concealing the 
foot, widest at the middle, rounded behind, and somewhat concave in 
front. Tentacles stout, truncated, grooved, and cylindrically tapering. 
Oral veil subtriangular. Branchiz tripinnate, with the pinne ar- 
ranged alternately. Foot oblong oval, rounded at bothends. Colour 
uniform vermilion. 
Length 1 inch. 
Under stones in the lower region of littoral zone. 
20. PLEUROBRANCHUS VARIANS. 
Oval, rather rugose, convex above. Mantle rounded behind, 
deeply sinuose in front, and margins slightly undulated. Ten- 
tacles arising from the head, curving laterally, deeply grooved below, 
truncated, cylindrically tapering, transversely lamellated. Eyes at 
their posterior bases. Oral veil large, convex in front, and much di- 
lating laterally, where it is deeply grooved. Mouth proboscidiform. 
Branchial plume simple, pinnate on the middle of the right side. 
Foot large, reaching the edge of the mantle laterally and behind. 
Colour varying ; some bright red, others lemon-yellow, or purplish 
brown, others again variegated with whitish; beneath paler than 
above. Shell on the anterior half of the body, concealed, small, fra- 
gile, pellucid, oblong-ovate, convex, and ornamented with wrinkles of 
growth. Nucleus posterior, more or less brownish. 
21. PLEUROBRANCHUS RETICULATUS. 
Oval, convex above, and covered with crowded depressed gra- 
nules, with multiangular bases. Mantle rounded behind and deeply 
sinuated in front, and repand, rather thin and undulated along the 
lateral margins. Tentacles arising from the lateral anterior portion 
of the head, approximating at their bases, stout, large, truncated, 
slightly swollen, transversely laminated, grooved in front. Eyes 
sessile, conspicuous at their posterior bases. Mouth proboscidiform. 
Veil large, granose above, triangular, and grooved laterally. Branchial 
plume single, simple, pinnate on the middle of the right side, free 
