130 



at different parts of the same shell, I found the following variations 

 The thickest part of the stratum of plates near the mouth of the 

 young Ampullaria, which were parallel to the lines of growth, may 

 be represented by the figures 25. At half the revolution of a whorl 

 backward, the same stratum was represented by 20, while the new 

 stratum beneath it measured 13; total 33. At a whole revolution 

 backward, the upper stratum measured 18, and the lower, or new one, 

 20 ; the older of the two thus appearing to decrease gradually in 

 thickness as the younger grew in substance. 



Upon examining, in a like manner, an adult specimen of Bulinus 

 oblongus, the proportions of the strata near the lip were as follow: — 

 outer 10, central 14, inner 17 ; total 41. But at one revolution they 

 were — outer 5, central 5, inner 19 ; total 29. 



In a great many univalves a remarkably rapid absorption of the 

 shell may be observed at about one revolution and a quarter backward 

 from the edge of the lip, and this appears to have been effected in 

 such a manner as generally to leave the involved whorl, where but 

 little strength is required, of a substance not much exceeding that of 

 writing paper. The portion of shell left is that which forms the inner 

 surface ; the whole of the outer and central strata, and the outer part 

 of the inner one, being removed. Jf a piece of this thin shell from a 

 Conus be examined by transmitted light, with a power of 280 linear, 

 we obtain a very beautiful view of the elongated prismatic cells, and 

 of the manner in which they frequently unite with each other, as re- 

 presented by fig. 2, PI. xv. 



In all the recent univalve shells which I have examined, the pris- 

 matic cells appear to be filled with a solid mass of carbonate of lime. 

 When they are fractured across their axes, as represented at PI. xv. 

 fig. 2, a, they do not present such surfaces as might naturally be 

 expected from crystallized bodies, but have an uneven, rough, and 

 somewhat spiculated appearance. Upon examining these organs 

 from the inner surface of a fossil ( Voluta nodosa) from the London 

 clay, I found that when broken at right angles to their axes, they re- 

 solved themselves into exceedingly minute fibres, so that each broken 

 end resembled a worn-out camel's-hair pencil. If these pencil-like 

 ends be crushed in a little water, on a slip of glass, and viewed by 

 transmitted light, with a power of 1000 linear, we find innumerable 

 transparent filaments, which appear to have a cylindrical form ; but 

 this point cannot be determined with certainty, from their extreme 

 minuteness. They vary in size ; but one of about the average dimen- 

 sions, which I measured, was the 30 ooo of an inch in diameter, and 



