154 Pelagian Mollusca collected on a [no. 2, new series, 



marked transversely with broad undulating sulci or farrows. 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 AtlantaFigs. 6 and 7. The animal is endowed with nata- 

 tory powers like the Pteropod 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 p£nultimate 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 



