Skptember, 191^ 



KNOWLEDGE. 



353 



as lenses of long enough 

 focus to form an image 

 in the grating above. The 

 others stop short, either 

 form no image or weaker 

 ones represented by the 

 apparent rings. In only 

 a few instances, however, 

 are these irregularities of 

 structure seen, and not 

 often is there an appear- 

 ance of rings, even when 

 not otherwise filled. The 

 spread slide from which 

 the photographs were 

 taken contains hundreds 

 of forms, but only in some 

 half-dozen are those varia- 

 tions apparent. The rest, 

 on the outer side, look 

 as in Figure 387, referred 

 to above. Yet the few- 

 exceptions supply the key 

 to the structure of the 

 Pleurosi^ma valve. 



The loan of a spread 

 slide by a friend has sup- 

 plied the writer with the 

 materials for the greater 

 part of the rest of his 

 article, Triceratium faviis 

 there being one of the 

 leading features: upon one 

 of the specimens, and 

 others after that, he 

 stumbled upon what he 

 thinks is a surprising dis- 

 covery. The surprise also 

 is that no one had ever 

 made it before. He has 

 started enquiries and 

 satisfied himself that the 

 knowledge up tc now re- 

 mained as his own sole 

 possession. Yet no other 

 genus of diatoms has been 

 so much written about. 

 Triceratium, like other 

 families of distinction, 

 possesses a history, and 

 was one of the first of the 

 kind to find a historian. 

 It was also the earliest 

 upon which was dis- 

 covered what is now- 

 known as secondary 

 structure. 



The genesis of this 

 family is first recorded 

 by Mr. Brightwell, F.U.S., 

 in the July number of 

 The Quarterly Journal of 



Fir,i:RE 392. T. fai-iis, from the centre of the same valve. 

 MaL'iiification — direct— three thousand diameters. 



iMi.iKi. :■''.<. 7". './i /I-,. Ii'iiii ir.i-;iiiMit .'I another valve, 



showing iR-arly all the tibnls torn away, and the covering 



over the hexagons wanting, where marked. Magnified two 



thousand diameters. 



Figure 394. 1 ,,- 



neath sound. 



..: hixagons under- 

 .\laL,'iutication the same. 



Microscopical Science for 

 1853, the title, " On the 

 Genus Triceratium, with 

 Description and Figures 

 of the Species." In the 

 beginning of the article 

 he says: "The genus 

 Triceratium, with several 

 other species, was estab- 

 lished by Ehrenberg in a 

 memoir communicated by 

 him to the Berlin Academy 

 in 1839-40. He founded 

 it uixm t\\'0 species, T. 

 faviis and T. striolatum, 

 the former of which is 

 generally taken as the 

 tvpe of the genus." A 

 plate accompanies the 

 article. There is no men- 

 tion of secondary struc- 

 ture by Mr. Brightwell, 

 only of " larger or minute 

 cellules"; what is now- 

 known as the frarnework 

 of hexagons. A second 

 article from the same 

 pen appears in the July 

 number of the same 

 joiunal for 1856, in- 

 creasing the number of 

 species, and is accom- 

 panied by another plate. 



The next article in the 

 sanie journal for July, 

 1858, is by Dr. Wailicii, 

 of the Belgian army, on : 

 " Triceratium and some 

 near allied forms," accom- 

 panied by a plate. There 

 for the first time we find 

 a hint, and a very small 

 drawing of secondary 

 structure in this diatom. 

 There is also an allusion, 

 somewhat obscure, to the 

 possibility of discovering 

 similar structure in other 

 genera of diatoms. Speak- 

 ing of T.fimbriatum (only 

 a variation of T. favus.). 

 Dr. Wallich says: "Each 

 hexagon in the St. Helen 

 form being, not merely a 

 simple depression depen- 

 dent upon the mode in 

 which the siliceous ele- 

 ment is secreted by the 

 inner cell membrane on 

 its inner surface, but a 

 deep hollow cell, with 

 perpendicular sides of 



