164 PNEUMONOBRANCHIATA. 



(Pfeiffer, t. 2. f. 23.) : when full grown; the smaller 

 shells are always rather thicker. It is always known 

 from H. cespitum of Drap. by the spire being lower 

 and the umbilicus wider. Mr. Jeffreys speaks of one 

 with a more produced spire found in lona, Western 

 Islands (Linn. Trans, xiii. 339.), but I have not seen 

 any that agree with Draparnaud's species. 



Lister, in his anatomical plates (t. 2. f. 10.), gives 

 some details of the anatomy of this species. 



5. ZONITES Montf. (Zonites.) 



Animal with an elongate depressed foot, and a large 

 produced central spiral body, covered with (and 

 contractile into) a depressed or hemispherical, thin, 

 shell with flattish spire, and a large lunate mouth, 

 with thin simple lips, which are neither thickened 

 nor reflexed ; the tentacles are four, the two lower 

 ones small and club-shaped. 



The animal can entirely withdraw itself into the 

 shell, and this genus is at once known from the former 

 by the thinness and generally polished state of the 

 shell, and also by its being depressed and destitute of 

 any internal rib round the edge of the mouth. 



It is intermediate between the Helices and the fo- 

 reign genera Stenopus and Nanina of the family Ari- 

 onidcs. The animal also resembles the latter in some 

 respects, but wants the gland on the end of the foot. 

 It is very probable that other peculiar characters will 

 be found when the animals of the different species of 

 HeUcidce have been described and compared together, 

 as Mr. Nunneley has so excellently well done with the 

 species of slugs. 



