294 DESCBIPTION OF [Polyfiastrica, 



often seem in motion. Two corpuscles unite by Zygosis {i. e. 

 conjugate) and produce, between them, a smooth, globose young one. 

 Genus TKirLOCEHAS (Bailey.) — Frond binate; segments straight, 

 much elongated, with numerous whorls of knot-like projections ; 

 ends of the segments threc-lobcd ; lobes bidentate. A genus closely 

 allied to Bocidium, differing in the three-parted ends of the seg- 

 ments. 



T. verticillatum (Bailey) = Bocidium verticillatimi (Ralfs.) 



T. Gracile (Bailey.) — Slender, with whorls of rounded projections. 



SECTIOIf II. — Navicflacea.. 



This section, together with the Echinellea, and the Lacernata of 

 Ehrenberg, are equivalent to the Biatomacea, Biatomece, or CymieUece, 

 of other authors. In the following observations on structure, habit, 

 &c., it is proposed to comprehend all the forms of Bacillaria, exclusive 

 of the Bcsmidiacea. 



The individuals of tlie Biatomea are variously spoken of as testules 

 {tesfnld') or frnstiiles; and also, when forming members of a compound 

 band, chain, or filament, or of other aggregated mass, as — segments, 

 coi-puscles, and sometimes loricse. But the term lorica, though often 

 used as equivalent with frustule, rightly expresses merely the 

 exterior coat, or shell which determines the form of the individual, 

 and is in this section always siliceous. 



This lorica, in its multiplied and often veiy beautiful forms, con- 

 sist sof an outer clear or hyaline, colourless, siliceous coat, and of an 

 inner more or less coloured one, considered to be of a softer and 

 more organic constitution. Nagcli supposes a mucilaginous film to 

 exist on the inside of this second coat : and Klitzing also speaks of 

 a third element, displayed when recent frustules are dried, and 

 especially after their having been heated to redness, as an opaque or 

 brownish space, extending itself, not unfrequently, over a considerable 

 portion of the lorica. To this material the name of cement is applied 

 by Klitzing, as he supposes it to be the connecting material of the 

 valves or portions of the lorica, and also of frustules when united. 



