( 853 ) 



XXI. — On some Photographs of Broken Diatom Valves, taken by 

 Lamplight. By Jacob D. Cox, LL.D., F.E.M.S. 



{Bead 12th November, 1884.) 



In a series of articles recently published* I gave tlie typical 

 examples of numerous observations which I considered to warrant 

 the following conclusions : — 



(1) The diatom shell is usually formed of two laminae, one or 

 both of which may be areolated, and may be strengthened by ribs 

 which have been described both as costae and as canaliculi. (2) The 

 normal form of the areolae is a circle, and these when crowded 

 together take an hexagonal and sub-hexagonal form. (3) Tlie 

 areolae are properly pits or depressions in the inner surface of one 

 of the laminae, so that when two laminae are applied together, the 

 exterior surfaces of the shell thus formed are approximately smooth, 

 and the cavities are within. (4) The apparent thickening on the 

 exterior of the lines bounding the areolae in some species, as 

 Eupodiscus argus, &c., is not in contravention of, but is in addition 

 to the formation above described. (5) However fine the dotted 

 marking of diatom valves may be, the evidence from the colour of 

 the spaces between the dots, and of the dots themselves, supports 

 the conclusion that they follow the analogy of the coarser forms in 

 which both fracture and colour are found to prove that the dots are 

 areolae, and the weaker places in the shell. 



I have now sent to the Society, through the courtesy of Mr. 

 Mayall, a parcel of photographs of broken diatom shells made by 

 lamplight. They are intended as a contribution to the evidence as 

 to the structure of the diatom valve, and were prepared under the 

 strong belief that the study of broken specimens has not been 

 followed up with the systematic care which their instructiveness 

 would justify. 



In my investigations, extending over a considerable number of 

 years, it has been my babit to mark by the Maltwood finder and 

 enter in my note-books the more suggestive examples of broken 

 valves which I found, and especially such as seemed to throw useful 

 light upon the question of structure. How to make this evidence 

 available was a somewhat troublesome problem ; but it is one which 

 recent improvements in dry gelatine plates for photography seem 

 to have solved very happily. Other duties prevented my using 

 the daylight hours for work of this kind, and I was for some time 

 deterred Irom attempting to photograph by lamplight by the fear 



• Atoer. Mori. Micr. Journ., v. (1881) pp. 45-1), 00-9,85-9, 104-9; aee thi« 

 Journal infm, p. 94:^. 



