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The structural arrangement in bivalve shells appears to be subject 

 to a greater degree of variation in different genera than we observe to 

 be the case among the univalves. 



Modiola papuana, when fractured parallel to the lines of growth, 

 and viewed as an opaque object with a Lieberkuhn, and a power of 

 150 linear, exhibits, on the interior surface, a thin stratum of columnar 

 basaltiform cells, at right angles to the natural surfaces of the shell, 

 while the upper surface appears dense, uniform, and apparently coin- 

 posed of numerous thin laminae, parallel to the natural planes of the 

 shell. In a fracture at right angles to the lines of growth, the co- 

 lumnar arrangement could not be distinguished, probably from its 

 being composed of plates of elongated cells having their planes pa- 

 rallel to the fractured surface. In Area Note, when fractured parallel 

 to the lines of growth, we observe plates of prismatic cells crossing 

 each other at right angles, and very closely resembling the mode of 

 arrangement of the prismatic cells in the greater number of univalves. 

 On examining Venus decussata, I found two distinct modes of ar- 

 rangement to prevail. From the inner surface towards the outer one 

 about half the substance of the shell is composed of numerous, thin, 

 membranous, and calcareous strata, which exhibit a dense, uniform 

 structure, when fractured either at right angles or parallel to the lines 

 of growth, but which are readily separated by maceration in weak 

 hydrochloric acid. The outer half of the substance of the shell, 

 when fractured in the direction of the lines of growth, presented the 

 appearance of numerous, basaltiform, columnar series of cells. When 

 fractured in a contrary direction, this stratum proved to be composed 

 of plates of elongated prismatic cells, having their planes at right an- 

 gles to the surfaces of the shell ; the lines of cells nearest to the 

 outer surface curving outwards, and those adjoining the inner one 

 curving inwards, so as to form fan-shaped plates of single series of 

 cells, crossing the lines of growth edgewise. This mode of arrange- 

 ment of the prismatic cells is not peculiar to Venus, but is found also 

 in other genera of bivalve shells. In the fragment of a large fossil 

 bivalve, which I took from the gault formation of Eastwear Bay, near 

 Folkestone, I found them in a peculiarly favorable state for examina- 

 tion, as the whole of the piece of shell separated readily into single 

 plates of fan-shaped structure, which exhibited this organization in a 

 very beautiful manner. Upon examining these plates when immersed 

 in water, by transmitted light, with a linear power of 300, I observed 

 that they were furnished with a vast number of canals, which appeared 

 to be of a similar description to those occurring in Haliotis, Ostrea, 



