384 ANIMAL NATURE OF DIATOMEtfl. 



D. mtreum, D. Uyalinum^ in the genus Fragillaria ; those 

 striated with elliptico-lanceolate lateral surfaces (Diatoma 

 vulgare, D. mesodon, D. tenue, D. mesoleptum,} in Odon- 

 tidium. There would remain, as distinctly generic, 

 the only two species which have capitate extremities on 

 their lateral surfaces, (D, elongatum, D. Ehrenberffii.} 

 These two proposed unions would be justified on both 

 sides, for whilst the Odontidia have forms little dif- 

 ferent from Diatoma, the Diatomte are little different 

 from Fragillarice. But I wish it to be well understood, 

 that it is not my object to propose changes in the nomen- 

 clature or systematic arrangement of DiatomeaB. I only 

 intend to institute an organographic examination of the 

 proposed genera and groups. And organ ographically 

 we shall probably have more important distinctions than 

 those of form between Diatonue and the Fragillarice ; 

 the latter have constantly two apertures at each extremity 

 of the principal surfaces, which seem to be wanting in 

 Diatomce, without adverting to the resemblance between 

 the DiatomtjB tenue var. dimotum and the very singular 

 Bacillaria. 



The character by which the last four genera (Denticula, 

 Odontidium, Fragillaria, Diatoma,} are collected together 

 into one family, namely Fragillarieae, is the conformity of 

 the two primary surfaces ; nor do I know how the genus 

 Meridian is excluded, even by the minutest characters. 

 Kiitzing, indeed, cites the affinities with Himantidia 

 among Eunotiea?, with Diadesmis among the Naviculeae, 

 with the various genera of Striatellese and Tabellarieae 

 among the Vittatae. The relation appears to us rather one 

 of analogy than of affinity, being the polypariform asso- 

 ciation of many individuals conditions which associate 

 together almost every form ; and by this rule Melosira 

 might be allied to Fragillaria. As to internal substance, 

 Kiitzing says it is originally uniform in the shield of all 

 Fragillarieae, then divided into minute particles, mixed 

 with oily globules, or contracted into a spherical mass. 



