APPENDIX. 519 
mediate space. The aperture is suborbicular, not thickened 
at the outer margin as in the Rissoe, and almost entire. 
The umbilicus is conspicuous. Axis 5, diameter = uncie. 
Animal.— Ground colour of the palest muddy yellow. 
Mantle even with the shell. Muzzle short, smooth, thick, 
though flat above, cloven at the end, mouth beneath, with a 
vertical fissure. Tentacula finely setose, divergent, flat, rather 
slender, gradually tapering from base to point, which is 
rounded, hyaline-white, suffused with minute snowy pomts ; 
eyes large, but rarely exserted, placed on low prominences at 
the external angles of the tentacula. The foot has three 
phases; when at rest it is rounded anteally, and still more so 
posteally ; when the animal swims it becomes narrow and 
rounded to a similar extent and form at both ends; on the 
march it is arched convexly in front, not auricled, and pos- 
teally has a blunt lancet-shaped termination; the sole is 
deeply impressed with a longitudinal central line; the oper- 
cular lobe is short, but laterally alated, and carries, near the 
extremity, a grossly spiral, yellow, rissoidean, subcircular, 
corneous operculam, which has been described as not spiral. 
This is an error. In clean examples under the microscope, 
the depressed nucleus and usual spiral turn may be traced ; 
a dozen examples have been examined, and though obsoletely 
impressed, they are clearly paucispiral. There is no distinct 
cirrhus springing from the operculigerous lobe, but in some 
specimens I have fancied I saw a very short blunt one. 
Sometimes the poimt of the main sole is bifid for a very short 
distance. The animal is most abundant on the littoral levels 
at Exmouth; it marches with great rapidity, and is vivacious 
and free; on the march the eyes are always under the shell, 
as are usually the muzzle and foot, the ends of the tentacula 
only being visible; it carries its shell at an angle of 70°. 
I have elsewhere stated that this species can scarcely remain 
in the genus Rissoa, but these various rectifications Induce me 
to withdraw that opinion ; it really appears to have as many or 
more rissoidean attributes than some of the species admitted 
into that genus. 
