470 ANIMAL NATURE OF DIATOME^E. 



55. Tabellaria. — Bacilli adnati obsolete stijntati, 

 demum semivoluti, concatenati, inferrupte loyigitudinaliter 

 vittati ; a latere secundario ventre et apicibus injlati. 



The variation of form in the frustules [bacilli) of this 

 genus is very remarkable. On this account it becomes 

 so very difficult to distinguish between the two principal 

 species [T. Jloccidosa, T. fenestrata) of this genus, com- 

 mon in fresh water throughout Europe ; and the synonyms 

 become so complicated and obscure. 



Hence Kiitzing rightly sought the limits between the 

 two species in the fine character of alternate vittse in 

 T. Jloccidosa, and opposite vittee in T. fenestrata. With 

 regard to these vittse, I observe that they are alike arranged 

 in fine stria3, which continue without interruption even 

 where the vittse are alternate. Their alternation in T.Jloc- 

 culosa is not at all constant, and Hassall, too, makes this 

 remark. 



After a long and attentive observation, I believe I have 

 convinced myself that the structure of the Tabellariece is 

 something different from that described and figured by 

 our author. The primary surfaces have a rectangular 

 figure ; the secondary are linear, or rounded merely at 

 the extremities (as I find in authentic specimens of T. 

 fenestrata, from Jurgens), or swollen circularly in the 

 middle, or at the ends, as seems constant in T. floccidosa. 

 It follows that the form is that of a cylindroid, strongly 

 compressed, or of three smaller cylinders, the central 

 one larger than the others, unitecl together by parallel 

 walls, and placed at a distance less than the diameter 

 even of the lateral cylinders ; I never obtained a 

 sight of any aperture. The internal cavity is variously 

 divided by incomplete diaphragms. When one only 

 exists, as is frequent in T. fenestrata, it is clearly seen to 

 be furnished with a large central perforation, and to con- 

 tain a canal, which, running along the internal wall of 

 the primary surfaces to where the diaphragm detaches 

 itself, comes to communicate with another similar canal, 

 hollowed in the free margin of the diaphragm itself. When 



