THE OPERCULUM. 51 



layers to the outer edge of its larger and last-formed end ; but 

 when it has to reproduce this organ, the opercular mantle having 

 reached a certain size, it proceeds to cover its surface with a new 

 protection in the most easy and rapid manner, and, commencing 

 from a more or less central spot on the surface, enlarges the 

 surface covered b}^ adding new matter to the entire circumference 

 of the first-formed part ; it continues this process without waiting 

 to make the operculum as thick and solid as the one which was 

 lost, until it reaches the size of the original, moulding itself on 

 the opercular mantle, and adapting its form to the form of the 

 throat of the aperture of the shell which it has to close. The 

 change of form in the front of the restored and mended oper- 

 culum is caused by the parts being moulded on the existing 

 opercular mantle — consequently they have not the narrow front 

 part which is found in the normal form caused by that part 

 having been formed when the animal had this part of a small 

 size ; and as it increases in size the Avhole opercular mantle 

 moves forward, leaving the small tip of the operculum free, and 

 useless to the animal, and, therefore, not necessary to be repro- 

 duced when the operculum is re-forined in the adult age of the 

 animal." 



The operculum presents modifications of structure which are 

 so characteristic of the subgenera as to be woi'thy of particular 

 notice. It consists of a horny layer, sometimes strengthened 

 by the addition of calcareous matter on its exterior. 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 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. 

 The operculum is described as — 



Concentric, when it increases equally all round, and the 



nucleus is central or subcentral, as in Paludina and Ampul- 



laria (Ixxiv, 16). 

 Imbricated, or lamellar (ii, 35), when it grows only on one 



side, and the nucleus is marginal, as in Purpura, Phorus, 



and Paludomus. 

 Claw-shaped, or unguiculate (ii, 341, with the nucleus apical 



or in front, as in Turbinella and Pusus ; it is claw-shaped 



and serrated in Strombus (iii, 49). 

 Spiral, when it groM^s only on one edge, and revolves as it 



grows ; it is always sinistral in dextral shells. 

 Paucispiral, or few-whorled (ii, 37), as in Littorina. 



