102 MALACOZOA. GASTEROPODA. PULMOBRANCHIATA, 



an internal spiral calcareous plait, attached to an elastic 

 pedicel, and closing the cavity when the animal has 

 withdrawn. 



1. Clausilia perversa. Dusky Clausilia. 



Shell slender, turrito-fusiform, rather glossy, slightly trans- 

 parent, with from ten to twelve little convex, transversely sul- 

 cato-striate, distinctly separated turns, the first two shining, 

 smooth, forming an obtuse apex, the rest gradually enlarging 

 to the penultimate, the last three faintly striated spirally, the 

 ultimate narrowed, with two wide grooves ; the aperture sub- 

 ovate, narrowed near the posterior end, which forms a rounded 

 angle, the margin thick, continuous, reflexed, elevated from 

 the columella, on which are two plaits, the posterior near the 

 angle, thin, and running out upon the lip, the anterior thicker, 

 less elevated, and not running so far out ; the colour deep 

 chestnut-brown, or blackish-brown, that of the peristome brown- 

 ish-white. Length half an inch, breadth from a twelfth to an 

 eighth. 



The shell varies considerably in form, as well as in colour. 

 Some individuals are much thicker and more fusiform than 

 others. The number of spiral turns is sometimes nine, often 

 ten, not unfrequently eleven, sometimes twelve. The colour 

 is reddish-brown, dusky-brown, or blackish-brown. Indivi- 

 duals occurring in dry places, much exposed to the sun, have 

 the shell thicker, and of a grey colour, as if weathered ; and 

 many have streaks of grey. The peristome, which is always 

 thickened, entire, raised, more or less reflexed, and whitish, 

 varies in form, being roundish-ovate or ovate, more or less 

 narrowed toward the posterior end, sometimes oblong, or 

 subquadrate. The varieties which I have observed in the 

 mouth, in specimens gathered in the same spot, are the follow- 

 ing : A single thin plate on the collumella near the posterior 

 angle of the mouth, (the shell elongated, narrow, thin, and 

 less prominently streaked). A thin posterior plate, and a 

 less elevated but slender posterior plate, scarcely apparent, and 

 not reaching the mouth. The posterior thin plate, a thick an- 

 terior plate not reaching the mouth, and a third concealed 

 plate formed by the dorsal fold of the shell. Two plaits, and 

 between them a very slender parallel plait. Two slender plaits 

 between the two larger. Three plaits, the middle one bipartite. 

 Very often the anterior plait appears thus bipartite, which 

 depends upon a prominent fold of the columella, more or less 

 apparent. Often also there is a transverse rib internally above 



