On the Structure and General form of Shells. 205 



The forraation of shells takes place by the deposition of suc- 

 cessive layers of shelly matter, secreted from the surface of the 

 mantle of the animals on the internal surface, and more especially 

 on the circumference of the already formed part, so as gradually 

 to increase its size and form in a more or less depressed or elon- 

 gated cone.'^ The smallest and outermost of the layers, or that 

 which is first formed, of this shelly cone, I call the nucleus ; and 

 the position of this part leads to the division of shells into two 

 great groups ; for when it is central, and the sides are alike, the 

 cone, and consequently the shell is symmetrical ; and it is not sym- 

 metrical when the nucleus is lateral and the side dissimilar. The 

 symmetrical shells are either straight as in Cleodora, recurved as 

 in Emarginula, incurved as in Patella, or convolute^ that is twisted 

 parallelly on the same axis, as in Nautilus. The non-symmetrical 

 shells are more or less spirally twisted, or in other words obli- 

 quely revolving round an imaginary axis, called the columella : 

 the shells in this case are called spiral, while the separate revo- 

 lutions are termed vchorles or whirles ; the whole, except the last, 

 being taken collectively as the spire. When, however, the spire 

 has become flat, or nearly flat, so that the axis is very short, and 

 the shelly cone (i. e. the whorles) very rapidly increase in size, 

 the shells are termed discoidal or sub-convolute ; the revolutions or 

 whorles are rarely distinct from each other as in Scalaria, but are 

 usually more or less compressed and soldered together, so that the 

 conical appearance becomes less distinct ; in this latter case their 

 inner sides in revolving come together, either so closely as to form 

 a solid columella, or are more or less distant and leave a space, 

 which according to its size, is either a perforation or an umbilicus, 

 or more properly a hollow axis. In some of the non-symmetrical 

 shells, the shelly cone enlarges so rapidly, that room is scarcely 

 left for it to become spiral, as in Pileopsis, Bulla aperta, Aplysia, 

 and especially Urabella ; but these may readily be distinguished 

 from the symmetric shells. 



The application of a new layer to the edge of the inner surface 



* This latter word 1 use in its most extended signification, as a body which 

 gradually increases in diameter, from its apex to its base, let it be round, com- 

 pressed, or even angular; straight or oblique. 



