ANIMAL NATURE OF DIATOME^i. 407 



Now with regard to the family of Cocconideae I can 

 only repeat what has been said of the genus Cocconeis, 

 that it presents a new type of organisation, differing 

 from the preceding, summarily, in this, that the temno- 

 genesis is effected transversely in the direction of the 

 body, though vertically in respect to the point of attach- 

 ment ; in other words, that in these the surfaces become 

 superior and inferior, which in the others were lateral. 



21. ACHNANTHIDIUM. Individua simplicia, singularia 

 vel binata, libera ; a later e primario linearia genuflexa. 



Admitting it to be proved that in the species of this 

 genus (A. microceplialum, A. delicatulum] there positively 

 exists a median aperture in one of the lateral surfaces 

 and not in the other, and that two perforations exist 

 at the extremities of the primary surfaces, as stated in the 

 definition of the order and in that of the family : admit- 

 in g this, we should still have to decide whether the un- 

 certain relations of these characters to other families, 

 and their inconstancy, will give us. any right to erect a 

 distinct genus on principles so slight and precarious. 

 This is, indeed, a systematic experiment that is not 

 sufficiently established on an organographical basis. 



22. ACHNANTHES. Individua solitaria vel binata vel 

 numerosa in fascias plus minusve elongatas transversaliter 

 conjuncta, stipite laterali adnata. 



In the want of striae three species (minutissima, exilis, 

 parvula,} present great analogy of form with the pre- 

 ceding. In one of these (parvula] there is wanting the 

 characteristic angular bending, for which reason it be- 

 comes very similar to Odontidium and Diadesmis. The 

 other ten species (striata;} differ only by very slight 

 characters from each other. Besides the organic differ- 

 ence between the two secondary surfaces, the constant 

 median aperture of the inferior or ventral, (Ehrenberg,) 

 and besides the process of duplication, which may be 

 studied, in all its details, in the Achnanthides better than 



