On the Form, Growth, and Construction of Shells. 251 



The " carrier- shell" (Phorus) cements other shells, and 

 stones, and fragments of coral to the growing lip, and they re- 

 main firmly attached, forming a spiral round the suture. In 

 this case, the hold of the mantle must be strong, or the ani- 

 mal must remain quiet for a while every time it adds a fresh 

 bit to its grotto. 



The section of any pearly shell exhibits an immense suc- 

 cession of fine and smooth layers. If polished or worn ever 

 so little, these laminre will be cut through, and their edges will 

 present a series of parallel lines. In the nacreous shell of 

 Haliotis, the layers are corrugated, so that a single layer might 

 serve to give the pearly effect. In porcellanous shells, the 

 entire structure is composed of layers of cells, much metamor- 

 phosed, arranged in three distinct strata, the direction of each 

 of which is different (see Plate, p. 245, Fig. 5, transverse section 

 of Gonus tesselatus, Born) . When seen in section, each stratum 

 is found to be composed of many vertical plates arranged some- 

 times transversely, sometimes lengthwise, according to the 

 genus. Pearly shells and fibrous shells, such as Pinna, are 

 like aragonite, biaxial, and prismatic, when examined with 

 the polariscope. Porcellanous shells, like calcite, are rhombo- 

 hedral and uniaxial. 



The prismatic structure of certain shells is well illustrated 

 by the fossil Hijypiwitidce from the cretaceous beds ; the an- 

 nexed Woodcut (Fig. 13) taken from the author's paper on 

 Hippurites (Quart. Journ. Greol. Soc, 1855, vol. xi., p. 47, 

 Fig. 12), represents part of the rim of Radiolites Mortoni, of 

 Mantell, from the lower chalk of Sussex. (Traced from the 

 original specimen in the Museum of the School of Mines, 

 Jermjn Street.) 



Fig. 13. Portion of the shell-wall of the lower valve of Madiolites Mortoni, 

 Mant. Lower Chalk, Sussex, a, the outer edge ; h, the inner edge ; v, v, 

 dichotomous impressions. 



A structure analogous to the chambered shell of the cepha- 

 lopod occurs in the thorny oyster, or Sjiondylus ; in aged speci- 

 mens, the shell, instead of increasing in size, becomes 

 thickened in its interior by the addition of inner layers of shell, 



