202 Conchology. 



CONCHOLOGY. 



:no. v. 



Of the formation of the Umbilicus, Protuberances, 

 4*c. We have hitheno considered only the general formation of 

 shells. In the present section we shall treat of some other cir- 

 cunastances which produce variations in their external fi2:ure. 

 Such, for instance, is the formation of the umbihcus, of spines, 

 tubercles, ribs, and other protuberances. 



Umbilicus. Univalve shells, which are furnished with a reg- 

 ular spire, may be divided, with regard to their form, into four 

 classes; namely shells having a disk, cyhndrical shells, turbinated, 

 and ovoid or egg-shaped shells. These four terms are the most 

 common, which spiral univalve shells assume, and they depend on 

 the manner in which the turns of the spire are applied to the com- 

 mon axis, and the difference in their arrangement. They derive 

 their primitive figure from the small shell while it is yet included 

 in the egg, and probably from that of the external organs of the 

 animal, which is contained in it. But, although all univalve shells 

 may be referred to one or the other of these four principal forms, 

 they exhibit a great variety of shghter shades of difference. Let 

 us now see in what way it may be conceived that the bodies of the 

 animals which inhabit univalve shells, give them a spiral form. 

 If we can suppose that from the first production of these animals, 

 when they begin to be developed, the fibres of one part of the 

 body, such as those of the external surface, are longer than those 

 of the opposite surface, it is obvious that the body of the animal 

 continuing to increase, according to its original tendency, will as- 

 sume a cur\ed form, the concave part of which will be on that 

 side where the fibres are shortest, and if the long fibres on the 

 external surface, and the short fibres on ihe internal surface, con- 

 tinue to increase in the same proportion, this must give the body 

 a spiral form; but in this case, the different convolutions of wliich 

 the animal is composed, will be in the same plane, and can only 

 apply to a small number of shells included in the first division, 

 namely, those which are characterized with having a disk. 



The convolutions of the spire which are described by the shell 

 of univalve testaceous animals, and the body which serves as a 

 mould for these, are disposed in different planes. Some other 

 cause, therefore, must operate in producing this deviation. Be- 

 tween the two surfaces of the body of the animal, which is sup- 

 posed to be furnished, with fibres of different lengths, it is easy 



