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in a direction about equally oblique to the planes of the whole of the 

 plates of prismatic cells, (PL xv. fig. 1, b). From this description 

 of the alternating mode of arrangement of the plates of cells, it will 

 be obvious that if, on examining a shell, we find a section parallel to 

 the lines of growth, with the plates of the central stratum agreeing 

 in their direction with those lines, while those of the outer and inner 

 ones are at right angles to them, we shall find the apparent direction 

 of the plates exactly the reverse in each stratum if we view a second 

 section of the same shell at right angles to the lines of growth. These 

 modes of arrangement of the tissues are invariable in the same spe- 

 cies and genus, as far as I have had an opportunity of judging ; and 

 it is somewhat remarkable, that of the eight genera examined, four 

 assumed one mode of arrangement and four the other. Thus, upon 

 viewing sections parallel to the lines of growth of the shells, I found 

 that in Cypraa, Cassis, Ampullaria and Bulinus, the plates of the out- 

 er and inner stratum were parallel to the lines of growth, while those 

 of the central one were at right angles to them. On the contrary, in 

 Conus, Pyrula, Oliva and Voluta, the plates of the outer and inner 

 stratum were at right angles to the lines of growth, and those of the 

 central stratum parallel to them. And it is worthy of remark, that the 

 porcellanous shells do not agree in the mode of arrangement of their 

 tissues, as we find Cypraa opposed to Conus, Oliva and Voluta, and 

 agreeing with Cassis, Ampullaria and Bulinus. 



The production of each stratum appears to be effected in regular 

 succession from the inner surface of the shell. Upon examining a 

 young specimen of Ampullaria fasciata, I found that at the extreme 

 margin, and to about the eighth of an inch backward, there was but 

 one stratum of cells, the plates of which were parallel to the lines of 

 growth. At about the eighth of an inch inwards, a new stratum of 

 cells, with plates at right angles to the lines of growth, commenced, 

 and rapidly increased in thickness as it receded from the lip, until, at 

 about half the revolution of a whorl backward, it had attained a 

 thickness equivalent to about one fifth the entire substance of that 

 part of the shell ; and at one revolution backward it was very nearly 

 equal in thickness to the layer above it. 



There are some discrepances in the relative degrees of thickness 

 of these strata, at different parts of the same shell, which are inte- 

 resting, and from which it may be inferred that absorption, as well as 

 secretion, probably takes place in these structures according to the 

 necessities of the animal. 



Upon measuring the relative degrees of thickness of these strata 



