ATLAS. 29 



Genus gasteropteron, Meckel. 



Head-disk triangular, with the eyes sessile on its upper 

 surface. Body ovate, bursiform. Branchial plume 

 exposed, on the right side of the body. Foot produced 

 on each side into a very large, rounded, fin-like lobe, 

 which, in repose, is folded over the back. 



Syn. Parthenopia, Oken. Sarcopterus, Rafin. Opi- 

 ptera, Rajin. Gasteroptera, Blainv. 



Ex. G. Meckelii, Kosse, pi. 58, fig. 6. 



This genus has been classed by some Zoologists with 

 the Pteropods, and has been described by Chiaje as a 

 species of Clio. One species only is known, an inhab- 

 itant of the Mediterranean. There is no shell, and the 

 side-lobes of the foot form, by their union, a transversely- 

 ovate expansion, retuse and slightly emarginate in front, 

 and produced in the middle ; by the aid of this fin- 

 like organ the animal is enabled to swim freely about; 

 when, however, it wishes to crawl, the side-lobes are 

 elevated and applied against each other over the back, 

 in which condition the animal somewhat resembles an 

 Aplysia. 



Genus atlas, Lesueur. 



Animal divided into two parts united by a peduncle, 

 the posterior oval, the anterior circularly dilated, with the 

 margins ciliated, and with a pair of small, auriform, ten- 

 tacular lobes above, and a very small, distinct foot below. 



Ex. A. Peronii, Blainville, pi. 58, fig. 7. 



This genus is not very well known. M. Lesueur, who 



VOL. II. f 



