Kaviculaeea,'] iNFrrsoniAL animalcules. 363 



in the water and turns upon itself ; detached frustules have a pro- 

 gressive movement. The granular contents are green or yellowish 

 when young, but reddish brown when old. The most evident manner 

 of propagation is dorsal self-di\dsion. In many species growth con- 

 tinues both diu'ing and after self-division, but in F. striatula increase 

 of size ceases after division. 



In Hooker's British Flora, the characters thus stand, — " Frustula 

 forming plane, pseudo -articulated, densely striated, fragile filaments, 

 separating at the striae (not cohering at their angles.) Named from 

 their fragile character." 



This genus gives name, and is the type of the family FragilaricB of 

 Kiitziug, which also includes the genera Benticula, Odontidiwm, and 

 Dkitoma. 



Mr. Ralfs observes, (Ann. [N'at. Hist., 1843.) "This genus is 

 nearly allied to Biatoma ; the chief distinction between them is, that 

 in Biatoma the frustules cohere at the angles, and in FragiUria do 

 not. In one species of the latter this difference does not exist ; it 

 seems therefore an intermediate form." 



" The puncta at the ends of the frustules, in this and other genera 

 of the CymheUea;, I have never found any reason to believe to be 

 openings, as many naturalists have-oupposed. Mr. Borrer informs me, 

 that in Fragilaria pectmalis they are the terminations of slight 

 grooves." 



Besides multiplying by fission, described by Ehrenberg, the fr'us- 

 tules of Fragilaria also conjugate and produce new forms. 



The followuig strictures by Kiitzing, on some of the specific dis- 

 tinctions of Ehrenberg, employed in this genus, occur in a note to 

 the description of Fragilaria ca-pucina (Kiitz.) 



" It varies much in length and breadth, the single frustules as 

 well as the bands. The contents also are subject to the same 

 variations as those of other Biatotnece. Nevertheless, Ehrenberg has 

 made use of these variations in the institution of several species. 

 P. 1 7, f. 1 is a tolerably common variety, which always appears 

 in very long bands. These bands are often twisted, and, owing to 

 this, appear smaller at some parts than at others, seeming to be 

 broader when lying flat, and smaller when at an acute angle. The 



BB 2 



