410 THE NUCLEUS OF THE SHELL. 



whorls, as in the Pyramidellse, or placed in an oblique posi- 

 tion with regard to the succeeding one, as in Voluta papu- 

 losa and some other species. 



The nuclei of many shells of different genera have not 

 the same characters as their parent shells ; thus, the nucleus 

 of the Tritons has a short anterior nick instead of an elon- 

 gated canal, and is very like a minute Buccinum. Some 

 retain the generic, but not the specific character of the group 

 to which they belong ; thus the nuclei of the Volutes in 

 general have the pillar slightly plaited, but the young of 

 Voluta musica has only two or three plaits on its pillar, 

 while the adult has many. 



The nucleus forms the original apex of the valves of all 

 shells, whatever may be their form, and frequently re- 

 mains attached to them during all their periods of growth ; 

 this is particularly the case with the Volutes, in which, 

 from its large size and rounded shape, it has been called 

 the Nipple. It is also conspicuous on most of the species of 

 Dolium and on some Fusi, among the univalves ; and on the 

 apices of the valves of the Cyclades, and Cham a?, among the 

 bivalves. 



This part of the shell has not received the attention that 

 it deserves. It is largest in those shells the animals of which 

 are viviparous ; and is consequently very distinct in the Vo- 

 lutee, Paludinae and Cyclades. In the oviparous species it 

 agrees in size with the egg of the animal ; thus Achatina 

 octona, which has an egg nearly equal in dimensions to the 

 mouth of the shell, and Bulimus ovatus and B. bicarinatus, 

 which have large eggs, have large nuclei, the magnitude of 

 the nucleus in general rendering the top of the spire blunt. 

 Some shells on the contrary, those, for instance, of the genus 

 Stylina, generally, and of the Pupa purpurea, have a very 

 long, slender, acute, turreted nucleus, but the form and size 

 of the eggs are in these cases unknown. 



The nucleus is found on examination to consist of two 

 very distinct parts or coats, the outer of which is membrana- 

 ceous or horny, and called the Periostracum, and the inner 

 hard and calcareous, and constitutes the shell. 



These two coats may be observed in all the stages of the 

 shell : they are generally very thin in the nucleus, and the 

 outer one is rarely distinctly visible in that state ; but it is 

 to be clearly seen (covered with five or more bands of hair- 

 like processes) in the very young Paludinae. In such shells 

 as are enveloped in the mantles of their animals, as the Dola- 

 bellas, Aplysiae and Bullaaa?, the outer coat or periostracum 

 is very thin ; it is, however, to be found in all shells, and 



