1# 



206 Mr. Gray's Conchological Observations. 



of the nucleus, usually leaves concentric striae or laminse on the 

 surface ; this sort of sculpture, whatever may be the position of 

 the cone, should be called concentric, or perhaps in spiral shells 

 transverse ; in bivalve shells it is often called antiquated^ a term 

 which is not objectionable ; but in the spiral univalves it has 

 generally been considered as longitudinal, which is evidently im- 

 proper. On the other hand, when the striae, ribs, or plaits, radiate 

 from the nucleus, and are continued across the concentric striae, in- 

 stead of being called transverse, the term usually employed, they 

 ought to be called radiate, longitudinal, or spiral, according to the 

 form which the cone assumes: from this deposition of shelly mat- 

 ter from the edge of the mantle, all the remarkable modifications 

 of the surface, as spires, tubercles, folds, foliations, digitations, 

 and branchy expansions take place ; they are formed on the mar- 

 gin left by the successive increase at the edge of the aperture ; 

 when they are the remains of the thickened and reflected lips, 

 they are called varices, from their resemblance to veins distended 

 with blood. 



The cavity of the shells, which varies according to the length 

 of the cones, is usually continuous ; but in some shells, owing 

 to the shortness of the body, the apex of the mantle forms one 

 or more shelly diaphragms, and then the shell is called cham- 

 bered ; these chambers generally remain attached to the shell, 

 but sometimes the upper ones fall off, when the shell is termed 

 decollated or truncated. 



I have before stated that the nucleus is the outermost and first 



* 



formed plate of the shell, it is consequently generally the apex ; 

 therefore the aperture, which is last formed, must be the base ; 

 let the cone be twisted how it may, this aperture is usually and 

 properly divided into right and left lip, according to the side of 

 the animal to whith it is next; and in the spiral or convolute 

 shells it is divided into the labium or columeliar lip, and the la- 

 bium, or outer or marginal lip. 



§ 2. On the Natural position of Shells. 

 The greatest confusion has existed with respect to the natural 

 pobition of shells, wiiich has unfortunately led to an indiscrimi- 



