RISSOA. 367 
rounded, deeply separated, rather oblique volutions, which are, 
particularly the body one, closely, but superficially, spirally 
striated ; the caducity of the striz renders this species very 
liable to become glabrous from attrition. The general colour 
of the external organs is a brilliant subhyaline white, but, 
though aspersed with minute opake snow-flakes, the transpa- 
rency is scarcely impaired. The mantle is even, and does not 
emit a process from the portion that lines the upper angle 
of the aperture. The head, when quiescent, is a short sub- 
cylindrical rostrum, quite smooth and rounded at the termi- 
nation; it is not tunicated, lobed, grooved, nor vertically 
cloven on the upper part, and on the lower area it forms a 
disk, which has not a distinct vertical fissure as in R. parva, 
but shows a fine crosial incision, which is the alimentary 
orifice; and within it are probably the corneous jaws and 
buccal apparatus, but I could not detect them, perhaps from 
being of the hyaline colour of the rostrum, which is so pel- 
lucid as to allow the intenser white canal or cesophagus lead- 
ing to the stomach to be seen through the walls. 
When the animal is on the march it often suddenly evolves 
the rostrum to double its usual length, at the same time ex- 
panding the termination into a large disk or finely dentated 
flattened rose, which it throws back on the margin of the 
upper point of the aperture, and then as quickly withdraws 
the extension to its usual limits: whether this curious man- 
ceuvre is part of the animal ceconomy, or of the nature of that 
which is sometimes seen in the typical Rissoe when disturbed, 
I cannot determine until more specimens are observed; at 
present, I think the action peculiar to this species. 
The tentacula are flat, strong, rather short, flake-white, 
smooth, gently attenuating and becoming minutely claviform 
at the tips, which are each clothed with six comparatively 
long, intensely aciculate setz ; the eyes are unusually large, 
black, and fixed on minute sub-semicircular lateral excres- 
cences at the external bases, and are so amalgamated with 
them as scarcely to present a prominence. The foot is a 
curious organ, being large, fleshy, anteriorly grooved, so as to 
_ form a slight labium, deeply indented in the centre, and pro- 
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