ANIMAL NATURE OF DIATOMEiE. 881 



8. Fragillaria. — Individua linearia IcBvissima sym- 

 metrice formata, in fascias vel rectus vel curvas, bicon- 

 vexas, arete conjuncta. 



The singular individuals which constitute the fiHfo|'m 

 aggregations of Fragillaria, are so like Naviculse that 

 there is often a doubt whether a particular species shopld 

 be referred to the Fragillarice or to the genus Biadesirf.is, 

 in which the filaments are formed of Naviculse possessing 

 the essential character of a median aperture in the lateral 

 surfaces. In fact, this doubt pervades all the new species 

 enumerated by Ehrenberg and by Kiitzing, with a note 

 of interrogation, among the Fragillarics. Thus the absence 

 of transverse striae does not seem a constant character, 

 or at least it seems that it ought not to be considered 

 essential, inasmuch as some of these doubtful species 

 {F. ? anceps, F. ? ampldceras) are striated. And Bailey 

 describes and figures numerous and evident strise, even in 

 F. pectinalis, merely observing that " it requires a high 

 magnifying power and skilful management of the, light 

 to render these apparent." In this genus, too, as well 

 as in the preceding one, and with a little more regularity, 

 we have frequently a cuneate form of frustule, resembling 

 that of Meridian. There is a highly important fact, in 

 an organographical point of vienr, presented by the 

 Fragillaria, in the variable length of the filaments as 

 compared with the invariable length of the frustules com- 

 posing it, a variation probably induced by age. In the 

 paucity of our positive knowledge respecting the succes- 

 sive development of these beings, we must take this fact 

 into consideration. The internal substance presents great 

 variety of distribution, as is shown by Kiitzin^'s reducing 

 to F. pectinalis all the species on which Ehrenberg had 

 founded his character deduced from this substance. The 

 four species safely referred to this genus Inhabit fresh 

 waters. 



8 {bis). Grammatonema, a genus hithei-to ill defined, 

 regarded by the author as intermediate between Fragil- 

 laria and Diatoma, and to which Kiitzing, besides the 



