If'avicuiacea.] infusortai, animalcules. 347 



DiADESMis sculpta ^i:: TahcUaria sculpta (Ehr.) — Margin striated. 

 ChiU. 



D. (?) Bacillmn = Kavicula BaciUum (Ehr.) — Segments striated, 

 linear- oblong, rounded at each end, central aperture large, oblong. 

 FossU. 



Genus Diatoma (Ag.) — Individuals linear, quadrangular, symme- 

 trical ; at first conjoined iia bands, but ultimately separated to such 

 an extent, that they are connected only by a more or less distinct, 

 jelly-like link, (isthnusj extending between their angles, — generally 

 the alternate ones. The filaments are flat or compressed. (Kiitz.) 



Biaioma differs from TaheUaria, (see that genus) Grammatophora^ 

 and Bhaldonema, by its frustules wanting longitudiaal bands f stripes J ; 

 and from the two first also, by the absence of a central aperture. 

 From Fragilaria it is distinguished by the angular and zig-zag con- 

 catenation of the finistulcs. Kiitzing says its filaments are often 

 attached. He makes it a member of the family Frcujilariece. 



Ehrenberg describes the species of Biatoma, (Kiitz.) some with 

 Fragilaria, others with Bacillaria. The latter is, however, nearly 

 equivalent with Biatoma, (Ag. ) as understood hy Kiitzing; 

 to convey his views, at the risk of repetition, the characters of 

 Biatoma presented by that obsciwer^re given. It may be observed, 

 that Kiitzing admits a genus BaciUarin, which is represented by one 

 species, Bacillaria paradoxa ; he places it in his family Surirelleee. 

 The reason he assigns for instituting this genus, is, that in Bacillaria, 

 the striae (seen in an end view) are interrupted in the middle by a 

 clear longitudinal band, which is not the case in Biatoma, 



The genus Biatoma of Mr. Ealfs, would appear to correspond with 

 Bacillaria of Ehrenberg, and with the genera Biatoma and Ba- 

 cillaria, of Kiitzing. Mr. Ealfs enumerates the following characters 

 and marks of distinction between it and allied forms. Filaments flat or 

 compressed, free frustules quadrangular, partially separating, and co- 

 hering by the angles, generally by the alternate ones. This genus ia 

 distinguished from Fxilaria, Striatella, Achnantkes, and Isthmia, by 

 its unattached filaments ; from Biddulphia by the angles not being 

 produced, and from Fragilaria by the connection of the frustules by 

 their angles, in a zig-zag chain. 



Sect. I. Forms quite smooth, 



A A 2 



