GASTEROPODA. 207 
mantle-lobes when the animal expands, acquire a glazed or 
enamelled surface, like the cowries; when the shell is deeply 
immersed in the foot of the animal it becomes partly glazed, as 
in cymba. In all other shells there is an epidermis, although it 
is sometimes very thin and transparent. 
In the interior of the shell the muscular impression is horse- 
shoe shaped, or divided into two scars; the horns of the crescent 
are turned towards the head of the animal. 
The operculum with which many of the gasteropods close the 
aperture of their shells, presents modifications of structure which 
are so characteristic of the sub-genera as to be worthy of particular 
notice. It consists of a horny layer, sometimes strengthened by 
the addition of calcareous matter on its exterior, and in its mode 
of growth it presents some resemblance to the shell itself. Its 
inner surface is marked by a muscular scar, whose lines bear na 
relation to the external lines of growth, and its form is unlike 
the muscular scar in the shell. It is developed in the embryo, 
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 rudimentary or obsolete. 
Fig. 70. Fig. 71. Fig. 72. Fig. 73. Fig. 74. 
The operculum is described as— 
Concentric, when it increases equally all round, and the nucleus 
is central or sub-central, as in paludina and ampullaria (Pl. IX., 
Fig. 26). 
Imbricated, or lamellar (Fig. 71), when it grows only on one 
side, and the nucleus is marginal, as in purpura, phorus, and 
paludomus. 
Claw-shaped, or unguiculate (Fig. 70), with the nucleus apical 
or in front, as in turbinella and fusus; it is claw-shaped and 
serrated in strombus (Fig. 76). 
Spiral, when it grows only on one edge, and reyolyes as it 
grows ; it is always sinistral in dextral shells. 
Pawcispirai, or few-whorled (Fig. 73), as in littorina. 
