lo4 Pelagian Mollusca collected on ct [no. new series. 
marked transversely with broad undulating sulci or furrows. The 
aperture is somewhat triangular and produced into three spines ; 
the shell had but a fragment of the animal adhering to it and was 
so much mutilated, that I cannot vouch for the representation being 
quite accurate. A little before 8 p. m. we took in the net two 
species of Atlanta Figs. 6 and 7. The animal is endowed with nata- 
tory powers like the Ptekopod but is possessed of a much more 
complex organization. It is classed with the Gasteropods and has 
like them a true foot which however is not fitted for progression in 
the usual manner on a plane surface, but is cleft into two fleshy 
expansions, which the animal uses very effectively as fins. Attached 
to the foot is a calcareous operculum, which in Fig. 7, shows very 
peculiar striae indicating that the nucleus of the operculum has 
been situated at the outer part of the aperture of the shell, from 
which the marks of increment proceed in successively increasing 
wavy lines towards the inner part of the aperture. 
It will be seen by a reference to Fig. 7, that the inner whorls of 
this shell appear as if chambered or divided into septa. This is en- 
tirely due to a peculiar joint-like formation of the viscera of the 
animal, visible through the transparent shell which is in reality 
unilocular. 
The animal has two tentacula and a proboscis, behind which 
are placed the eyes which do not project externally but are visible 
under a thin layer of integument. On dissecting out one, and sub- 
jecting it to a high magnifying power, I found it to possess a crys- 
talline lens of spherical form, partly imbedded in a black cushiony 
mass, which appears to serve the purpose of absorbing the rays of 
light and is somewhat analogous to the pigmentum nigrum in the 
human eye. The magnified eye in its dark chamber is represented 
in Fig. 7 a. The shell is transparent and colorless, discoidal, spiral, 
in shape not unlike some of the smaller species of Planorhis. An 
expanded keel is attached to the outer circumference of the shell 
and extends to the space between the last and penultimate whorls, 
which it connects together. The general form of the shell is ex- 
tremely elegant. Atlanta Fig. 6 of which we obtained only a 
solitary specimen, is a very remarkable and interesting shell, it is 
