GASTEROPODA. 149 



impression of a calcareous lid is left upon the shell most distinctly on both sides of the 

 aperture, and I have presumed fig. 9,/, to have been the operculum. 



Natica depressula. — Some small specimens in my cabinet, not more than a 

 line in diameter, passed under this name in my Catalogue. They are depressed in the 

 volution with a deep and distinct suture, that does not appear to have been produced 

 by erosion, and differ in that respect from the young of some well-determined species 

 that I have traced downwards to no greater magnitude. They must, however, for the 

 present, be considered of doubtful character. 



SiGARETUS, Adanson, 1757. 

 Stomatia. Brovme, 1/56 (fide Gray). 

 Cryptostoma. De Blainv. 1825. 



Gen. Char. Shell auriform, or ear-shaped, suborbicular, oblique, with a depressed 

 spire, and consisting of few and rapidly increasing volutions, striated or reticulated 

 upon the exterior; aperture much dilated, wider than long; peritreme disconnected 

 by the body whorl, with, generally, a small but open umbilicus, and a thin corneous 

 operculum. 



The shells of this genus are said to be entirely enveloped by the mantle of the 

 animal, although the exterior is covered with stripe or reticulations. It is distinguished 

 from Siomatia, Lamarck, by the texture of the shell, which in this is not nacreous, 

 and from Marsetna, to Avhich it approaches nearer in form, by its thicker and more 

 opaque shell, and by its having the inner lip spread over the body whorl, forming 

 an umbilicus. In Marsenia the left side is sharp and truncated, visible internally up 

 to the spire. 



All the known recent species of this genus are natives of warm climates, and 

 two or three species have been found in the Eocene formations. It is first quoted from 

 the Devonian period. 



1. SiGARETUS EXCAVATUS. S. Woocl Tab. XV, fig. 11. 



S. Tesfd minutd, aurifonni, tenui,fra(jU'i; sjxird acuta, promimdci ; anfradihus iribus, 

 depressis, obliqins, supra convexis, siibtits plaiiatis, imbricatis ; ad marginem obtuse angu- 

 latis, transversim undulato-siriatis, striis exilissimis ; aperturd pateiite ; umbilico magna, 

 profunda. 



Shell small, thin, and fragile, with a slightly prominent spire, and acute apex ; 

 composed of three depressed volutions, convex on the upper part, and flat beneath, 



