380 ANIMAL NATURE OF DIATOME^l. 



and yet, notwithstanding all this, it is included in the 

 grand division of Diatomece non vittata. The name of 

 any section and the character whence it is derived, need 

 not be constant when the section itself is founded upon 

 an assemblage of other characters, and its true charac- 

 ters consequently derived from affinity. But when this 

 character is isolated, it entirely loses all the systematic 

 value it can possibly have, so soon as it is wanting in 

 constancy. 



7. ODONTIDIUM. Tndividua quadrangula, a latere 

 secondario transversim striata, lanceolata, in fasciam 

 biconvexam arete conjuncla. 



The Odontidia are merely a filiform series of Denticula. 

 Whilst among the Himantidia there are promiscuously 

 enumerated species of which the individuals are conca- 

 tenated, and species with individuals merely geminate 

 (H. Arcus), here, on the contrary, two genera are distin- 

 guished by this character solely. And the same character 

 the cuneate form of the frustules by which the genus 

 Meridian is distinguished from Himantidium, is again 

 met with in Odontidium, but inconstantly and irregularly. 

 I do not wish to infer from this that these generic dis- 

 tinctions may not be appropriate, because in the paucity 

 of our knowledge respecting the intimate organisation of 

 these beings, there is no adequate support for this opinion 

 or the contrary ; but it seems to me a clear inference that 

 in an organological point of view these characters possess 

 little value. Then, the number of transverse striae varies in 

 the same species, probably according to age. Six spe- 

 cies inhabit fresh waters, principally alpine, one only (0. 

 syriacum) inhabits the sea, one (0? glans] is found 

 fossil, and one (Fragillaria grandis, Ehr.) is uncertain, 

 to which, as an uncertain species, we may add the Syrinx 

 annulata of Corda, which, if the figure (' Aim. Carlsb.,' 

 1833, tab. iv, figs. 45, 46,) be correct, has the singular 

 character of continuity of the transverse striae even over 

 the primary surfaces. 



