370 DESCRIPTION or {Tolygastrica. 



first- named naturalist makes two sections of this genns, according as 

 the segments are globular or elliptic, and keeled (The term keeled, 

 carinatus, is ai:)plied by Kiitzing to the opposite sides of each seg- 

 ment, which ai^pcar produced beyond the jB.rst or chief circle, or cut 

 off by a segment of another circle from it), or cylindrical and not 

 keeled ; calling the first Lysigonium, the second Gallionella. " This 

 genus," says Mr. Ralfs {Jinn. Nat. Hist., vol. xii, p. 347), "in 

 its cylindrical filaments, differs from the other Cyinhellm, and thus 

 connects them with the Coufervte; but it agrees with them in being 

 generally of a brown or yellowish colour when recent, and especially 

 in its siliceous filaments, and in the presence of striae ; characters 

 which sufficiently point out the propriety of its present situation 

 among the Diatomece. The filaments have no proper margins marked 

 by distinct characters, as in the other genera of Cymhellece (Diatomea;) ; 

 and the stria3, ■N\hcn present on the junction-sui'faces, are not trans- 

 verse, but radiated. 



"In the fii'st section of this genus, the species belonging to which 

 are generally' marine, the ends of the frustules are convex, and as 

 there are no distinct jimction-sui-faces, a moniliform appeai"ance is 

 produced. The central line is more strongly marked in this than in 

 the other section, and seems to dVfide the frustules into two equal 

 portions. It becomes broader, and at length double, and, ulti- 

 mately, an intermediate growth separates the two halves of the 

 frustule, which, during this process, do not increase in size ; but 

 when the intermediate space is equal to the diameter of the original 

 fi'ustulc, two new frustules are formed, by the addition of two hemi- 

 spheres on the inner sides of the separated portions. The outer sili- 

 ceous covering still remaining, the frustules are connected in pairs, 

 and appear like two globules within a joint, as they are charactciized 

 by Harvey in G. nummuloides, and by Cannichael in G. globifera. 

 The above description belongs more particularly to G. nummuloides, 

 but the process in the other species, in the first section, is the same ; 

 a series of changes, nearly similar, occurs in Isthmia. 



" All the species in the second section are found in fresh water. 

 The frustules are not united in pairs ; their junction-surfaces are 

 distinct, and nearly flat, and their central lines are probably furrows ; 

 hence, whatever parts of these lines occupj- the margin of the field 



