70 AKCADJS. 



snow-white ovarium attached to it, full of ova in 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 laminae 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 branchiae 

 hangs vertically, the upper part of it being 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 margin 

 into about fifty denticular points; 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 manoeuvres. 



This species is extremely common alive in the Exmouth 

 coralline region. 



Conchologists have exercised their ingenuity in convert- 

 ing some very trifling varieties into distinct species. The 



