194) SHELLS AND SHELL-FISH. PART I. 



flattened, without being absolutely reflected backwards. 

 Lastly, we venture to place a few small shells, having 

 something the aspect of Helicince* , as a fifth type, under 

 the name of Lucidella; they differ from the last by the 

 comparative smallness of the aperture, which has one or 

 two small teeth on the outer lip, but none on the inner : 

 these appear to pass into Thelidonta ; and in this man- 

 ner do we conceive the whole form a circle. Not 

 having yet completed our analysis of the three aberrant 

 genera, Piis'wdon, Tlielidomus, and Leiostoma, we shall 

 not at present attempt to designate their sub-genera. De 

 Ferussac has adopted a modification of Humphrey's 

 name of Lucerna (which he writes Lucena) for that 

 group, part of which we here call Leiostoma ; but how 

 far he is correct in placing the richly-coloured shells 

 upon his plates 12, 13, and 14. with that which we de- 

 signate as the type — may well be questioned. The dis- 

 torted body- whorl of Thelidomus, and its serrated outer 

 lip, render it analogous to Lucidella, from which it differs 

 altogether in the few and unequal volutions of its spire. 

 The most interesting genus, however, is that of Hemio- 

 don, which has a large dilated aperture, the spire scarcely 

 raised, and a gibbous prominence more or less developed 

 at the base of the outer lip. This is the last remnant 

 of the toothed aperture so common in this sub-family ; 

 and accordingly prepares us for the true Helicellce, which 

 compose the first genus among the HeUciu<v. Thus do 

 the LucERNiNyE foHu their circle of affinity, connected 

 on one side to the slugs by Leiostoma, which passes 

 into Vitrina, and on the other to the land helixes^ 

 as just stated. 



* Such as Helicina aureola, Zool. Journ. i. p. Ifi. f. 13. 



