68 JOURNAL OF THE [April, 



simply, in the others, smash them into bits so small as to be use- 

 less for purposes of investigation. 



This is so generally ; but in places enough of the structure is 

 left intact to establish its identity, and I need not say that, having 

 once found out what a chain is, there is no difficulty afterwards 

 in proving that a link is a part of a chain, even if we can find no 

 more than that link. In F. forjiiosum, as we have seen, we find 

 whole chains ; in the finer forms only occasional links, but unmis- 

 takably belonging to the same sort of chain. 



The first slides of F. angitlatum, Nos. 35, 36, 37, and 38, are 

 taken (three of them across the nodule) to show the different 

 curves of the two sides of the valve ; Nos. 35. 36, and 37 being 

 from the inner side, and No. 38 from the outer. It will be seen 

 that on the inner side the surface starts straight from each side 

 and curves down towards the median line, while the outer starts 

 straight from the median line and curves down towards each 

 margin. The outer, or convex side, like the corresponding side 

 of F. foriiiostiju, is the one from which all my torn examples are 

 taken, and is absolutely identical in character ; but beyond F. 

 decorum I have been unable to discover any diagonal layer imme- 

 diately underneath. The non-discovery of this may be due to 

 two reasons : it may not exist at all, or the two may be so close 

 together that even the little depth of focus of a wide aperture 

 may be too much to allow them to be separated, in which case 

 they would be microscopically non-existent. I have made 

 experiments to this purpose on F. formosicm with a dry apochro- 

 matic |-inch of .95 N. A , and found that the torn structure 

 could not be seen with it when lying at the usual distance above 

 the valve, but when the valve had sunk down — increasing the 

 normal distance between the two layers — they could be seen 

 readily enough ; indeed, when floated off the valve altogether 

 they could be seen with a one-inch of very moderate angle. I 

 can offer no evidence, therefore, of the existence of a double 

 layer on the outer side of the finer forms, although analogy may 

 tell us there should be one, and must leave it at present as not 

 proven. 



The first two slides of the inner side, Nos. 35 and 36, taken 

 on the same spot, will give you both squares and hexagons, and 

 you must take your choice between them ; for, having no exam- 



