RISSOA. 350 
with muddy yellow or orange above, but brighter on the 
under surface. The tentacula are very long, white, and 
slender, clothed with distinct horizontal setz from base to 
pot, which become more rare towards their termination. 
Eyes black, on minute yellow prominences at the external 
angles. Foot very long and narrow; the front portion is 
greatly extended on the march, subrotund, slightly labiated 
and scarcely auricled, terminating posteally in a lanceolate 
point; the centre of the sole has a depression, accompanied 
by a posterior medial line; on each side the upper surface, 
about the middle, is a well-marked line of lead-colour. The 
operculigerous lobe is very distinct, though not much alated 
anteally, and terminates in an expansive rounded membrane, 
which, hke the foot, is marked on each side with a blotch of 
dark lead-colour, but of greater width; these run into and 
almost unite on the upper surface, forming what some authors 
term a posterior dark spot. At the anterior junction of the 
foot with the body there is a transverse yellow or orange 
band, which is seen through the sole; the operculigerous lobe 
emits a very long and distinct caudal cirrhus, as much so as 
in R. parva, and it carries close to the posterior part of the 
foot-pedicle, a suboval, white, obliquely striated, paucispiral, 
corneous operculum. 
The animal in every respect represents a typical Rissoa; it 
is remarkably lively and free, and affords every facility for 
examination. It marches with great rapidity, and inhabits 
abundantly the coral zones of the South Devon coast at 
Exmouth, in an offing of six miles. 
I have stated in the ‘Annals of Natural History,’ vol. vi. 
p. 33. N.S., that I thought this species a coralline zone va- 
riety of R. parva; I withdraw that opinion, as I am satisfied 
of its distinctness. 
The preceding observations were written in 1851, but in 
the summer of 1852 I had the good fortune to meet with 
some rare unrecorded animals of the British Rissoe. To 
show that an account of these minute species is a desidera- 
