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many of them were not above half so large. Among these spicular 

 bodies there were small 'fragments of plates, formed of similar minute 

 fibres, disposed across each other at right angles, so as very much to 

 resemble a piece of fine linen. I could not ascertain whence this 

 curious tissue came, but from its frequent recurrence in the field of 

 the microscope, there can be no doubt of its being a portion of the 

 regular organic tissue of the shell. I have never found more than 

 three strata of plates in any univalve, nor in any part of the same 

 shell, however old or thick it might be. I examined the lip of a Cas- 

 sis tuberosum, which was half an inch in thickness, while the adjoin- 

 ing portions did not exceed one-tenth of an inch in substance. The 

 great thickness of the shelly matter in the lip was produced by the 

 extension of the outer and inner strata, while the intermediate one 

 was but little increased in thickness ; and the elongated cells, of which 

 the plates are composed, retained in each stratum their proper direc- 

 tions and modes of arrangement. In another case, at half an inch 

 within the lip of a very thick shell of Strombus gigas, although it ap- 

 peared to the eye as if there were as many as five or six distinct 

 layers of shell, yet, upon examination beneath the microscope, there 

 was but one stratum, exceeding half an inch in thickness, and in this 

 all the plates were parallel to the lines of growth in the shell. 



The structures, and their peculiar modes of arrangement, which I 

 have already described, are those which are common to the great mass 

 of univalve shells; but there are other forms of tissue belonging to 

 this division of our subject, to which I must now direct the attention 

 of my readers, and which vary in their characters to so great an ex- 

 tent from the larger number of univalves, and approach in the ar- 

 rangement of their cells and other tissues so nearly to the majority of 

 bivalve shells, as to render it advisable to treat of their peculiarities 

 at a point in our investigation as it were midway between the two 

 great divisions of our subject. The shells which best illustrate these 

 structural peculiarities are those which belong to the genus Haliotis, 

 and the one which I have more particularly made the subject of my 

 researches is Haliotis rubra of Lamarck. If we examine a surface 

 fractured at right angles to the inner and outer planes of the shell, as 

 an opaque object, by the aid of the Lieberkuhn, with a power of 300 

 linear, — whether the fracture be parallel to the lines of growth, or at 

 right angles to them, — we observe no oblique disposition of prismatic 

 cells such as prevails in all the other genera of univalve shells that I 

 have described ; but in either case a regular basaltiform columnar ar- 

 rangement of cells, whose lines of direction are at right angles to the 



