214 THE CANADIAN NATURALIST. [Vol. vi. 



" dorsal limb " is the opposite side of the operculum, in contact 

 with the dorsum. In some of the opercula there is a point 

 around which the surface markings are arranged concentrically ; 

 this is the " nucleus." 



The following species occur in the pebbles and boulders of a 

 coni2;lomerate which constitutes an important formation on the 

 south shore of the St. Lawrence below Quebec. The age of the 

 rock in which these pebbles are found, is not yet certainly de- 

 termined, but it is, at all events, near that of the Potsdam. 



H. COMMUNIS. — This species attains a length of about eighteen 

 lines, although the majority of the specimens are from ten to 

 fifteen lines in length. The ventral side is flat (or only slightly 

 convex) for about two-thirds the width, and then rounded up to 

 the sides. The latter are uniformly convex. The dorsum, al- 

 though depressed convex, is never distinctly flattened, as is the 

 ventral side. The lower lip projects forward for a distance equal 

 to about one-fourth or one-third -the depth of the sliell. In a 

 specimen whose width is three lines the depth is two lines and a- 

 half. 



The operculum is nearly circular, gently but irregularly con- 

 vex, externally and concave within. The ventral limb is seen on 

 the outside as an obscurely triangular, slightly elevated space, the 

 apex of the triangle being situated nearly in the centre of the 

 operculum. The base of the triangle forms the ventral margin. 

 This limb occupies about one-third of the whole superficies of the 

 external surface. The remainder, constituting the dorsal limb, 

 is nearly flat, slightly elevated from the margin towards the 

 centre. On each side of the apex of the ventral limb there is 

 a slight depression, running from the nucleus out to the edge. 

 On the inside there is an obscure ridge, corresponding to each 

 one of the external depressions. It is most prominent where it 

 reaches the edge. These two ridges meet at the centre, and 

 divide the whole of the inner surface of the operculum into two 

 nearly equal portions. 



The surface of the operculum is concentrically striated. The 

 shell itself in some of the specimens is covered with fine longi- 

 tudinal striae, from five to ten in the width of a line. The shell 

 varies in thickness in diflerent individuals. In some it is thiu 

 and composed of a single layer, but in others it is much thickened 

 by concentric laminae, and thus approaches the structure of a 

 Salterella. There are also fine engirdling striae, and sometimes 

 obscure sub-imbricating rings of growth. 



