24 



emarginate at tlie centre of the extremity, the lobes rounded. Mantle 

 free, and branchial cavity open. Foot with a spirai horny operculum, 

 angular at the upper part." 



I may add to these descriptions that Mr. Clark has lately stated that 

 the teutacula of Truncatella Montagui are " short, flat, broad, triangu- 

 lar, and diverge greatly, scarcely forming an angle of 25°. The eyes 

 are large and black, and have white prominent pupils, which visibly 

 dilate and contract. I have never observed such in any moUusk, 

 though similar ones may have escaped uotice ; they are plaeed a 

 little nearer to the base than the middle of their lower half, not on 

 pedicles, but quite flat on the centre of seniicircular expansions of 

 the outer side of the tentacles, with an external tendency. The 

 branchial plume is single, of an elongated, kidney-shaped figure, and 

 has the usual constriction or sinus at the end nearest to the heart ; it 

 can be deteeted with high povrers in sunlight, through the body 

 volution of pale, clear, thin shells." 



The eyes of Truncatella littorea " are precisely those of T. Mon- 

 tat/ui, and a similar white pupil is a singular coincidence." 



In conclusion, I may observe, that I regard the general form and 

 organization of the animal and shell of Truncatella as so peculiar, that 

 I have long considered it the type of a peculiar family, characterized 

 by the form of the lips and feet, the mode of walking, the short, 

 broad, diverging tentacles, the position of the eye and its pecuhar 

 form, and the tnmcation of the shell. 



On the other hand, the general form of the animal, the manner and 

 habitation of the genus Assiminia are so likę those of some of the 

 smaller species oiLittorina (which Dr. Leach uamed Sabancea), that 

 if it was not for the peculiar position of the eye on its long pedicel, 

 I should have been inclined to have considered it as a subdivision of 

 that genus, with very short tentacles and elongated eye-peduncles. 

 But Mr. Berkeley's observations have set that at ręst, as well as the 

 distinction between it and Truncatella ; for he shows that Assiminia 

 has lungs likę Cyclostoma, or rather Helicina, while the Littorinee 

 and Truncatellce have well-developed gills for respiration, likę the 

 greater part of the marine genera ; but the gills of Littorina and 

 Truncatella are very unlike one another, the gills of the former being 

 broad, short, laminar, and of the latter, single, ovate, and pectinate. 



P.S. — Messrs. H. and A. Adams, in the number of their work 

 issued since this paper was read, are so impressed with the pecu- 

 liarity of the combination of characters that the animal presents, viz. 

 a pulmonary respiration, spirai operculum, and terminai eyes, that 

 they have formed for the genus a suborder named Prosophthalma, and 

 a particular family, Assiminiadce : see Genera of MoUusca, 313. 



