102 



MAXUAL OF THE MOLLUSCA. 



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

 itself. Its inner sui-faee is marked by a muscular scar, whose lines bear no 

 relation to the external 'Knes of gro^\i;h, and its form is unlike the muscular 

 seal' in the shell. It is developed in the embyro, within the egs, and the 

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

 and concentric fomis fit the apertm-e of the shell wth accuracy, the others 

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

 large apertm-es (e. g. doUum, cassidaria, harpa, navicella), it is quite rndi- 

 meutaiT or obsolete. 



Fig. 63. 



Fig. 64 



Fig. 65. 



Kg. 67. 



The operculum is described as — 



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

 or sub-central, as in pal iidina and ampullaria (pi. IX., fig. 26). 



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

 nucleus is marginal, as in purpura, phoms, Knd. paludomus. 



Claio-shcqted, or unguicidate, (fig. 63, with the nucleus apical or in front), 

 as in turbineUus 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*. 



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

 times amount to 20 ; the number of tiu-ns which the operculum makes is not 

 determined by the number of whirls in the shell, but by the cui-vature of the 

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

 fit it constantly {Moseleij). 



It is said to be articulaied 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 volnia, 

 oliva, conns, mitra, and cancellaria, but absent in others ; it is (indifferently) 

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

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

 tJiium, it is tmdtispiral ox paucispiral. 



Some of the gasteropoda can suspend themselves by glutinous thi'eads. 



