298 Royal Society. 



its base an eye consisting of a globular lens with optic nerve and 

 retinal expansion. The foot was large and very mobile, but a vesi- 

 cular float has been observed only in MacyilUvrayia. 



The respiratory siphon was either a simple fold of the mantle 

 forming a temporary tube (Cheletropis), or a fold whose borders 

 were united through their whole length, leaving an aperture at the 

 end, as in MacyilUvrayia. 



A lingual ribbon with well-marked rachis and pleurae occurs in 

 all the species. Very perfect labial plates with closely- set dental 

 points arm the mouth in some instances, and probably in all. 



The little animals possessing in common the characters here de- 

 scribed, nevertheless fabricate shells so very different as to admit of 

 their division into well-marked genera. 



The author conceives that the obvious difference between the 

 pectinibranchiate type of respiratory organs and that observed in 

 the group of Gasteropoda now under consideration, affords sufficient 

 grounds for placing the latter in a distinct order by themselves ; and 

 as illustrations of it he proceeds to give some details of the structure 

 of the two species mentioned in the title of the paper, whose shells 

 have been already described by the late Prof. E. Forbes, and figured 

 in Mr. Macgillivray's ' Narrative of the Voyage of H.M.S. Rattle- 

 snake.' 



In MacyilUvrayia the disc of the foot is broad and connected by a 

 narrow attachment to the body just beneath the neck ; it carries an 

 operculum behind, and is cleft by a notch in front. A raphe ob- 

 servable in the median line, as well indeed as the whole character of 

 this part of the organ, seems to shadow forth the transformation of 

 the single foot of the Gasteropod into the wing-like expansion of 

 the Pteropod. 



After describing the labial plates and lingual strap, the eyes and 

 the branchiae, the author observes that the tubular siphon protrudes 

 from the shell on the left side and seems to indicate the coexistence 

 of a respiratory chamber with naked branchiae. 



The vesicular float, like that of Ianthina, noticed by Mr. Macgil- 

 livray, consists of an aggregation of vesicles varying both in number 

 and size in different cases. It is exceedingly delicate, and could not 

 be found in the specimens first obtained, having probably been de- 

 stroyed or detached from the foot by the force of the water running 

 through the meshes of the net with which they were captured. Its 

 coexistence with an operculum shows that it is not a modification 

 of the latter. 



Of the Cheletropis Huxley i, numerous specimens were found in 

 Bass's Straits and in the South Pacific, between Sydney and Lord 

 Howe's Island. 



After giving some details respecting the shell and the foot, the 

 author observes that the latter organ was destitute of float, at least 

 in the specimens he obtained, but was furnished with an operculum, 

 which, probably from its extreme thinness and smallness, had 

 escaped the notice of Professor Forbes. He then points out the 

 peculiarities of the respiratory apparatus. 



