6 REPORT — 1844. 



satisfactorily for the varying size of the reticulations as shown on a transverse 

 section of them, — some of the cells being cut across at their thickest, and some 

 at their thinnest part. The very small hexagons which are occasionally seen 

 in the midst of larger ones (fig. 7), are evidently the sections of prismatic 

 cells, which are coming to a pointed termination. Of this fact I shall pre- 

 sently make further use (§ 14). 



8. The great thickness of the basaltiform layers in many of the fossil 

 Pinnce (and their allied genera) renders them very favourable subjects for 

 examination of their structure, by a section at right angles to their surfaces. 

 It is then seen that the direction of the prismatic fibres is seldom quite 

 straight. In the same section they are often cut longitudinally in one part, 

 and obliquely or almost transversely in another. Hence, although it is 

 plain from the appearances shown on fracture, or by the disintegration of the 

 shell, that most of the fibres pass continuously from one surface to the other, 

 it is seldom that the whole length of them can be displayed in any one section, — 

 one set frequently passing off by a change of direction, and another coming 

 into view. Even to the naked eye, the curvature of these fibres is often 

 sufficiently evident in the large Pinnce and Inocerami ; a circumstance which 

 may, I think, be regarded as adding weight to the conclusion, that the pris- 

 matic character of the fibres is not to be attributed to crystalline action, but 

 to the form of the cells in which the calcareous matter is deposited. 



9. The general structure of the outer layers of the shell of Pinna (and, 

 as I shall hereafter show, of many other genera) may be thus desci'ibed : — 

 it consists of a stratum of prismatic cells, usually more or less hexagonal, 

 adherent to each other by their sides, and forming the surfaces of the layer 

 by their flattened terminations. Most of these cells pass continuously from 

 one surface to the other, so that their length corresponds with the thickness 

 of the layer ; but some of them end, by acute terminations, in the interior of 

 the layer, when its thickness is considerable (figs. 2 and 10). These cells are 

 filled with carbonate of lime, which give firmness to what would be otherwise 

 a soft membranous stratum. From the universality with which this kind of 

 structure, when it presents itself at all, forms the external layers of the shell, 

 and from the complete correspondence between the form and aggregation of 

 its cells, and those of the Epithelium covering the free surfaces of the other 

 membranes of the body, I think we are justified in regarding the prismatic 

 cellular substance of shell (which is the term by which I have designated 

 this kind of structure) in the light of a calcified epithelium. It would thus 

 correspond with the Enamel of Teeth, to which it is analogous in every re- 

 spect, save the character of the mineral deposit, and the much larger size of 

 the prisms. 



10. A more minute investigation of this structure throws some additional 

 light on the mode in which it is at first produced. When a thin section is 

 made of the shell of Pinna nigrina parallel to its surface, it exhibits a beau- 

 tiful reddish-violet hue by transmitted light, which is not, however, uniformly 

 diff'used over the Avhole section, some parts being commonly almost or com- 

 pletely colourless (fig. 1). This appearance is completely explained by the 

 examination of a thin section made in the opposite direction ; and it is then 

 seen that there is an alternation of coloured and colourless strata through the 

 whole thickness of the layer; so that the variations in the hue of the hori- 

 zontal section are due to the mode in which these strata crop out, one from 

 beneath another (fig. 2). If the section, however, should happen to traverse 

 one layer only, its hue will be uniform throughout ; and thus I have sections 

 of the same shell, taken from the same part of it, in some of which the whole 

 is colourless, whilst in others it is uniformly tinted. Now these facts are in- 



