102 



MANUAL OF THE MOLLUSCA. 



terior, and in its mode of gro\vtli it presents some resemblance to the shell 

 itself. Its inner sm-face is marked by a mnscnlar scar, whose lines bear no 

 relation to the external lines of gro\A'th, and its form is unlike the muscular 

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

 point from which it commences is termed the nucleus ; many of the spii-al 

 and concentric forms fit the apertm-e of the shell with accm-acy, the others 

 only close the entrance paiiially, and in many genera, especially those \\'ith 

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

 mentaiy or obsolete. 



Fig. 63. 



Fig. 64. 



Fig. 65. 



Fig. m. 



Fig. 67. 



The operculum is described as — 



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

 or sub-central, as inpaludina and anipidlaria (pi. IX., fig. 26). 



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

 nucleus is marginal, as in purpura, phonis, dx^^ palndomus. 



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

 as in turbinelh'.s and fusus ; it is claw-shaped and serrated in stromdus 

 (fig. 69). 



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

 always sinistral in dextral shells. 



Paueispiral, or few-whirled (fig. 66), as in Jitorina. 



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



Multispiral or many-whii'led (fig. 65) as in troclius, where they some- 

 times amount to 20 ; the number of tiu'ns which the opereuliun makes is not 

 determined by the number of whirls in the shell, but by the curvatui'e of the 

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

 fit it constantly {Jloseley). 



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 voluta, 

 oliva, conus, }nitra, and cancel/aria, but absent in others ; it is (indifferently) 

 horny or sheUy in the species of ampnllaria and naiica ; in pahtdina it is 

 concentric, in palndomus lamella)-, in valvata spiral ; in solanum and ceri- 

 thiHm,\i is multispiral ox paueispiral. 



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



