ON THE MICROSCOPIC STRUCTURE OP SHELLS. 7 



teresting, as proving, I think, beyond a doubt, that the filling up of these long 

 prismatic cells with carbonate of lime was not accomplished at one nisus ; 

 and that there must have been a succession of deposits, of which some were 

 tinted by the admixture of a coloured secretion, whilst others were left 

 colourless. The outer portion of each layer will of course be the part first 

 formed ; and the coloured layers are usually most numerous and deeply 

 tinted in its neighbourhood. 



11. The idea of a succession of deposits is borne out by a very curious ap- 

 pearance, which is presented by the two elements of the structure, when they 

 are separately examined. The prismatic cells of the decalcified membrane ex- 

 hibit a series of transverse markings at a small distance from each other, which 

 bear no small resemblance (as Mr. Bowerbank has remarked) to the transverse 

 strife of muscular fibre. These markings may be best seen by looking at 

 the sides of the cells, in a vertical section which has been decalcified by 

 dilute acid ; and they impart to the long prisms very much the aspect of the 

 scalariform vessels of plants (fig. 11). But they may frequently be well seen 

 in a horizontal section (with or without decalcification), when, as often hap- 

 pens, the direction of some of the prisms is somewhat oblique, instead of being 

 perpendicular to the plane of the section. Markings of a precisely similar 

 nature are seen upon the calcareous prisms themselves, both from recent and 

 fossil shells ; and they evidently correspond with those which the cell-walls 

 exhibit. 



12. These markings are attributed by Mr. Bowerbank to the existence of 

 a vascular network, by which he supposes each stratum of prismatic cells to 

 be surrounded. He thinks that a network of tubes, passing round each cell, 

 may frequently be seen in the decalcified membrane ; and that the slight 

 bulging inwai'ds, which the passage of the tube between the contiguous walls 

 of two cells will give to each of them, is the cause of the marking in question. 

 I cannot but think, however, that this view has been somewhat hastily 

 adopted. In the first place, we know of no instance in which vessels pass in 

 this manner through a cellular structure, except in the adipose tissue of 

 animals, to which the fabric of shell bears no resemblance. I have in vain 

 looked, in many scores of carefully-prepared specimens, for appearances 

 distinctly indicative of the passage of tubes between these cells ; but have 

 never succeeded. I can in any one, however, readily produce the appear- 

 ance figured by Mr. Bowerbank as a vascular reticulation, by throwing the 

 cut edges of the membrane a little out of focus. Moreover, if these tubes 

 have a real existence, they ought to be very evident in the shell, before decal- 

 cification ; in which I have never been able to find a trace of them, although 

 I have examined more than 100 sections, cut in various directions, of various 

 species of Pinna alone. When it is considered that the striae are seldom 

 more than 1 -5000th of an inch apart, and are frequently much less, it is 

 evident that there must be at least 5000 strata of this vascular network in a 

 layer of shell an inch thick. According to Mr. Bowerbank, these strata 

 communicate with each other by vertical tubes passing upwards and down- 

 wards from the angles of the reticulations. These also I have failed to see, 

 although I have used every variety of magnifying power and of method of 

 examination. I may mention also that, as will presently appear, I have 

 found numerous instances, in which a tubular structure of great delicacy is 

 readily discernible in Shell ; so that I am quite familiar with the appearances 

 which such a structure in Pinna might be expected to present. 



13. By submitting the cut edges of the membranous wall of the prismatic 

 cell to a high magnifying power, under favourable circumstances, I have 

 been able to discover what I believe to be the real cause of the transverse 



