428 FORMATION OF SHELLS. 



other ; if, however, the shell be cracked transversely to its 

 layers, the crystals will be found continued across the line 

 which separates them. An analogous structure exists in some 

 minerals, the Haematite for instance, the balls of which ap- 

 pear to be formed of separate concentric coats, but never- 

 theless when they are broken exhibit the crystals radiating 

 from the centre to the circumference without interruption. 



The plates of which the rhombic crystalline shells are 

 formed are deposited in succession, each gradually increasing 

 in thickness as the shell enlarges, and undergoing no varia- 

 tion in this respect after the deposition of the succeeding 

 coat has commenced. That the coats are deposited in regu- 

 lar succession, may be seen by examining the lip of any shell 

 which has been taken whilst the animal was increasing its 

 size. At this period the lip will be found gradually shelving 

 and becoming thinner from the inner to the outer edge, the 

 innermost part being formed of three, the next of two, and 

 the outer and thinnest part, which is always the first formed, 

 of only a single layer. This is best seen by making a section 

 of the lip of a S trombus or a Cone along one of the spiral 

 grooves, in which, if the polished edges be examined, the 

 layers will be distinctly seen. When the animal is about to 

 make its periodical stoppage of growth, the second, and 

 afterwards the innermost layer is deposited up to the edge 

 of the mouth, which is thus completed. 



In the Olivae, Ancillariae, and some Volutae, which have, 

 at all periods of their growth, a polished surface (now known 

 to be caused by their shell being more or less immersed in 

 the large foot of the animal), the outer layer, although 

 equally crystalline, is very thin. It is harder and much more 

 compact than the others, and between it and the central 

 layer, there is deposited an opaque, white, powdery film, 

 which often causes it to break off in splintery flakes, while 

 the rest of the shell separates into fragments, generally more 

 or less cubical, their form depending, doubtless, on the rect- 

 angular disposition of the laminae of which the plates are 

 formed. 



Some Olives, as Oliva utriculus, O. undatella, and O. acu- 

 minata, have an additional band, in structure and hardness 

 resembling the outer coat, forming a belt over the latter, 

 across the front of the whorls ; and some Ancillariae, as 

 Ancillaria marginata have also a similar belt placed on the 

 back of the volutions. 



When the animals of many of the shells of this structure 

 arrive at their full size, or when they form the successive 

 mouths of their shells at their periodical stoppages of growth, 



