102 MANUAL 'OF 'THE ' ftOLLTTSCA. 



terior, and in its mode of growth it presents some resemblance to the sheH 

 itself. Its inner surface is marked by a muscular scar, whose lines bear no 

 relation to the external lines of growth, and its form is unlike the muscular 

 scar in the shell. It is developed in the embyro, within the egg, and the 

 point from which it commences is termed the nucleus ; many of the spiral 

 and concentric forms fit the aperture of the shell with accuracy, the others 

 only close the entrance partially, and in many genera, especially those with 

 large apertures (e. g. dolium, cassidaria, harpa, navicella), it is quite rudi- 

 mentary or obsolete. 



Fig. 63. Fig. 64. Fig. 65. Fig. 06. Fig. 67. 



The operculum is described as 



Concentric, when it increases equally all round, and the nucleus is central 

 or sub-central, as in paludina and atnpullaria (pi. IX., fig. 26) . 



Imbricated or lamellar (fig. 64), when it grows only on one side, and the 

 nucleus is marginal, as in purpura, phorus, and paludomus. 



Claw-shaped, or unguiculate, (fig. 63, with the nucleus apical or in front), 

 as in turbinellus and fusus ; it is claw-shaped and serrated in strombus 

 (fig. 69). 



Spiral, when it grows only on one edge, and revolves as it grows ; it is 

 always sinistral in dextral shells. 



Paucispiral, or few- whirled (fig. 66), as in litorina. 



Sub-spiral, or scarcely spiral, in melania. PI. VIII., fig. 25*. 



Multispiral or many-whirled (fig. 65) as in trochus, where they some- 

 times amount to 20 ; the number of turns which the opereulum makes is not 

 determined by the number of whirls in the shell, but by the curvature of the 

 opening, and the necessity that the operculum should revolve fast enough to 

 fit it constantly (Moseley). 



It is said to be articulated when it has a projection, as in nerita 

 (fig. 67). 



Too much importance, however, must not be attached to this very va- 

 riable plate, as an aid to classification ; it is present in some species of vvluta, 

 olwa, conus, mitra, and cancellaria, but absent in others ; it is (indifferently) 

 horny or shelly in the species of ampullaria and natica ; in paludina it is 

 concentric, in paludomus lamellar, in valvata spiral ; in solarium and ceri- 

 thium, it is multispiral or paucispiral. 



Some of the gasteropoda can suspend themselves by glutinous thready 



