70 ARCAD A. 
snow-white ovarium attached to it, full of ovaim July. The 
mantle is open from the anus to the buccal aperture, its 
margins plain; there are no tubes, not even an orifice, but 
the rectum terminates within it by a simple perforation in the 
body. There are two branchial laminz on each side, brown, 
of an elongated triangular form, and fixed horizontally ; the 
broader part of them is at the short truncate or anterior 
end of the shell, from whence they gradually taper to the 
posterior termination, close to the debouchure of the rectum ; 
the upper lamina,—which is contrary to the usual plan of 
those organs in most of the Acephala,—is by far the largest, 
and entirely envelopes the under one of similar shape; they 
are finely striated on the outer surface, but more visibly on 
the inner. There are two labia or pendulous appendages on 
each side; the one nearest to the anterior end of the branchiz 
hangs vertically, the upper part of it bemg of a white, elastic, 
coriaceous substance as far as the middle, when it changes into 
a transversely striated brown narrow leaf-like lamina, the half 
of which is longitudinally doubled on the other portion; the 
second labium, or that nearest to the buccal orifice, coalesces 
with its counterpart on the other side, encircling the mouth, 
and then accompanies the other, on the same side, in a pen- 
dulous direction to the ventral region; it is long, linear, 
coarsely transversely striated as its fellow, and has also the 
one half folded on the other. This is a very singular struc- 
ture of these organs, but the foot exhibits a still greater de- 
parture from the ordinary forms of that organ; it is pale 
yellow, of a regular oval form, deeply serrated at the margim 
into about fifty denticular pomts; one-half the disk when 
within the shell lies folded on the other, but when in action 
it becomes a flat subcircular disk ; the motion of the animal, 
though progressive, is not direct ; it turns round as on a pivot, 
and its path describes an irregular ellipse. The animal is shy, 
and requires continual attention to observe these manceuvres. 
This species is extremely common alive in the Exmouth 
coralline region. 
Conchologists have exercised their ingenuity im convert- 
ing some very trifling varieties into distinct species. The 
