GASTEEOrODA. 207 



mantle-lobes when the animal expands, acqtiire a glazed or 

 enamelled surface, like tlie cowries ; when the shell is deeply 

 immersed in the foot of the animal it becomes partly glazed, as 

 in cymha. 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 no 

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

 the muscular scar in the shell. It is developed in the embryo, 

 within the Q^g, 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, harjpa, navicella), it ia 

 quite rudimentary or obsolete. 



JFig. 70. 'E.g. 71. Fig. 72, 



The operculum is described as — 



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

 is central or sub-central, as in.jpaludina and amjpuUaria (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 

 paludoTnus. 



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 stromhus (Fig. 76). 



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

 grows ; it is always sinistral in dextral shells. 



Faucispiral, or few-whorled (Fig. 73), as in littorina. 



