Conchology. 203 



to conceive two other surfaces directly ojDposite to each other, an 

 upper and an under surface, each of which is included between 

 the two preceding surfaces, but of smaller extent; and it is easy to 

 conceive farther that these two latter surfaces are so formed, that the 

 fibres of the one are longer than the corresponding and opposite 

 fibres of the other. According to this structure, the body of the 

 animal will tend to that surface on which the fibres are shortest, 

 and thus describe, duiing its developement, a spiral line in differ- 

 ent planes, in proportion to the difference of tension between the 

 superior and inferior surface of the body, as well as between the 

 lateral surfaces. 



The form of the shell depending on the exteraal form of the 

 body of the animal, the miibilicus, which is a different cavity 

 from that of the opening of the shell in which the animal is con- 

 tained, and which is seen on the interior surface of some shells, 

 in the centre of the convolutions of the spire, depends entirely on 

 the plane on which the animal has formed the additions to its shell. 

 If the plane of these convolutions has been directed roimd a con- 

 ical or ehptical axis, and each convolution of the spire be more 

 or less distant towards the centre of the shell from this hollow 

 point, a shell may be thus formed, whose umbihcus will be more 

 or less open, according to the greater or less degree of separation 

 which the animal must give to the convolutions of the spire, cor- 

 responding to its structure. An opposite effect will be observed, 

 if the increase of the convolutions of the spii'e is supposed to take 

 place round an axis which is so small as to permit them to come 

 in contact with each other. In this case no cavity will be formed 

 in the centre, no appeaiance of an umbilicus will be seen. But 

 if we conceive that the animal, in enlaying itself, turns round a 

 solid curved figure, in place of a conic axis above alluded to, and 

 that the end of this solid is at the summit of the shell, it is obvious 

 that an opening or an umbilicus of the shape of this solid, will 

 be formed in the shell. 



Ribs. The longitudinal elevations which are observed on uni- 

 valve shells, which run in a transverse direction to the successive 

 growth of the convolutions of the spire, have been denominated 

 rarices, by Linnsus, in allusion to the dilated veins on the bodies 

 of other animals. They are composed of one or more elevations, 

 usually arranged in a line parallel to the axis of the shell, and some- 

 times slightly oblique. They consist of the same substance as 

 that of the rest of the shell, but are thicker and always more ele- 

 vated than the suj'face of the convolutions of the spire on which 

 they are placed. To explain the manner in which these convo- 

 lutions are formed, we may examine the opening of all land shells 



