418 DIATOMACE^. 



Derivation. From pepis, a portion or particle. 



This is a very natural genus, differing from Fragilaria in 

 the cuneate form of the frustules in the front view, and atte- 

 nuated outline in the end or lateral aspect, and from Stylaria 

 in the number of frnstules, which cohere, and which form 

 more or less considerable portions of circles. 



1. MERIDION CIRCULARS Ag. 

 Plate XCVI. Figs. 16. 



Char. Frustules very variable in size, broad margin, punc- 

 tated. Articulating surfaces clavate, strongly striated; 

 the ends of the strife appearing as puncta along the edges 

 in the front view. 



Meridian circulare Ag., Consp. Diatom, p. 40. ; Kiitz. Sy- 

 nop. Diatom, in Linntea, 1833, p. 558. f. 37. ; Harv. 

 Manual, p. 205. Meridian vernale, p. 207. t. 16. f. 2. 

 Bailey's American Bacillaria, in Amer. Jour. Sci. Jan. 

 1842, pi. ii. f. 42. Echinella circularis Grev., in Wer- 

 nerian Society, vol. iv. p. 213. pi. viii. fig. 2., 1822; Scot. 

 Crypt. Fl. vol. i. t. 35. M. circulare Ralfs, in Annals, 

 vol. xii. p. 458. pi. xviii. f. 1. ; Jenner, in Flora of Tun- 

 bridge Wells, p. 206. 



Hob. Common throughout Great Britain, in pools, &c. 



The frustules of this very beautiful production exhibit all 

 the variety of sizes which characterise the different threads 

 of the Fragilarice, and this resulting from the same cause, viz. 

 the lateral growth of the frustules. In the smaller and nar- 

 rower frustules the end view is broadly clavate ; in the larger 

 and deeper frustules it is but slightly so, indeed almost linear. 

 I have occasionally met with frustules which have been very 

 perceptibly curved ; this form possibly arising from the 

 spiral disposition of the lamina in the more perfect spe- 

 cimens. 



Colour yellowish brown, assuming a green tinge in drying. 



