404 DIATOMACEiC: 



In the end view the frustules have been aptly compared to 

 the qiiatrefoil of a gothic window : in the smaller filaments in 

 this aspect, they are about as long as broad, in the larger one 

 and a half times as broad as long ; and each has about seven 

 well-marked stri{:e5 a central straight stria which extends from 

 one marginal lobe to the other, and three curved striae be- 

 lonfrino; to each of the front and back lobes. 



69. TABELLAKIA Shut 



Char. Frustules quadrangular, fusiform, cohering hy their 

 angles, traversed in the centre hy a longitudinal canal, 

 marked with transverse * stricB, interrujpted in the centre by 

 the canal. End view destitute of markings. 



Derivation. From tahella, a letter ; which the frustules re- 

 semble in form. 



The term Bacillaria is applied by Ehrenberg to the spe- 

 cies of this and the following genus, as well as to a curious 

 production wholly different from either genus, the Bacillaria 

 paradoxa, and to which sj)ecies I propose that the genus 

 Bacillaria should now be confined. By Agardh, the ap- 

 pellation of Diatoma is given to them. Mr. Shuttleworth 

 has proposed the division of the genus Diatoma into two 

 genera, in the propriety of which I fully concur ; for one of 

 these Mr. Shuttlewortli reserves the name of Diatoma, that 

 of the other is derived from a species named by Ehrenberg 

 Bacillaria tabellaris, and which species is typical of the genus. 



1. Tabellaria flocculosa. 



Plate XCIV. Figs. 9, 10.; Plate XCVI. Fig. II. 



Char. Frustules iii young spscimens as long as broad, in older 

 specimens their breadth exceeds their length by tico or three 

 times. Striae from one to seven on each side. In end 



* The word timnsverse has here a meaning different to that generally 

 assigned to it ; it here has reference to the width of the cell or filament'^ 

 and not to its depth. 



