Kaviculacea.] infusobial animalcttles. 297 



In the Navicuke, Schlcidcn believes he has proved this by direct 

 observation. Mr. Ralfs, indeed, states, that, in JVavicuIa and other 

 genera of Biatomaceee, the frustnles are often truly binate, but wo 

 question whether they are actually so, except when fission is pro- 

 ceeding. Throughout his learned essay on the cell-nucleus and 

 cell-formation, Niigeli treats of the Liatomacea as uni-ccllular 

 organisms. 



Each frustule generally presents four sides, or planes, which, un- 

 fortunately, have been variously designated. Ehrenbcrg has used the 

 terms dorsum, venter, and lateral surfaces or sides, but which do not 

 in all instances represent homologous surfaces. Oftentimes he has 

 called a convex surface, simply on account of its convexity, the 

 dorsum ; and a concave one, from its concavity merely, the venter, or 

 ventral surface. 



Kiitzing has endeavoured to amend this nomenclature, by calling 

 those sides which have no central opening, but through which self- 

 division occurs, the primary sides ; and the other two, the secondary 

 sides. These secondary sides are further distinguished into right and 

 left, when the frustule lies on a primary side. The left is often 

 concave, and the right convex, but mostly the two are alike. As a 

 general inile, the primary sides con^cspond with the so-called lateral 

 surfaces of Ehi-enberg, and the secondary sides, respectively, with the 

 dorsum and venter of that author. 



The Kev. "W. Smith has the following remarks on this subject, 

 which illustrate the usage of most English ^Titers: " Late wiitera 

 have found, in the process of self-division, circumstances to fix the 

 terminology applied to the Diatomaceous fr'ustules, and use the words 

 'primary sides,' when speaking of those portions where the inter- 

 position of the new haK-frustules occurs; the term secondary sides 

 being applied to the general surface of the valves ; others employ the 

 words front and lateral view in the corresponding senses. I shall 

 adopt the latter terms as more generally applicable; the primary 

 side, as employed by the writers alluded to, frequently including 

 portions of the frustule which belong to the secondary surfaces, brought 



into view by the convexity of the valves With Mr. Ealfs, 



and other English writers, I would use the tQvm. front view to denote 

 the aspect of the frustulcs, Avhen the connecting membrane and 



X 



