490 DESCEIPTION OF [Poli/ffastrtca. 



Genus Htalosiea (Kiitz.) — Frustules (bacilli) tabular, quadrate, 

 in chains, stipitate laterally, interrupted by vittate ; vittse alternating, 

 connected by veiy fine lines with the centre. It belongs to Kiitzing's 

 family Striatellce. All the species are new. (This and the two follow- 

 ing genera belong to sub-sec. 2.) 



H. minutissima. — Shortly stalked, concatenate, segments quadrate, 

 half detached from one another, very small. Length 1 -5700th. 



H. delicatula. — Shortly stalked, concatenate ; segments quadrate, 

 half detached, minute. Length 1 -2640th. 



H. redangula. — Shortly stalked, concatenations imperfect, segments 

 sub-quadrate, rectangular, larger (P. 17, f. 23.) Length l-1380th. 



H. ohtusangular. — Stipes long, filamentary or sub-concatenate; 

 segments quadrate, angles obtuse, large (P. 17, f. 29.) Length 

 l-1440th. On Bryopsis, Venice. 



Genus Isthjiia. — Frustules combined so as to form irregularly and 

 imperfectly branched filaments, attached ; frustules trapezoid or 

 rhomboid, cellulose, oblique, articulated to each other by a short 

 neck-like process — isthmus — near which an apparent pore is observ- 

 able. Division obliquely transverse. Lorica siliceous, undestroyed 

 by heat or acid. 



"Frustules always more or less oblique; the adherent (basal) 

 fnistule, the isthmus forms the stipes. The frustules are turgid, and 

 the reticulations of the central portion smaller than those of the 

 sides. On each side of this central portion are seen the lateral siir- 

 faces, with their reticulations and striae arranged transversely. As 

 these lateral surfaces are turgid and compressed, they appear, at first 

 sight, to form part of the front of the frustule. This may be observed 

 in some slight degree in Diatoma vulgarey That such however 

 is not the case " will be foimd, on a careful examination of the 

 exterior ; but it is still more evident on a view of the interior of 

 a fractured frustule, when the junctions of the sides, with the central 

 portion marked externally by mere lines of separation, form 

 interaally projecting plates or rims." 



" This genus is easily disting-uished, by its cellulose structure, from 

 all the neighbouring genera, except BidAulyhia and Amphitetras ; but 

 these differ in their frustules not being oblique, and in having all 

 their ends elongated. Kiitzing makes it a member of the family 

 BidclulpMece 



