1896.] The Classification of Diatoms. 533 
most nearly to a natural one because it is based on characters 
having physiological significance. It is based primarily on 
the structure of the endochrome, and secondarily on the method 
of forming auxospores and the general shape of the frustules. 
Van Heurck does not employ this system in his Synopsis 
because of the large number of fossil specimens and those from 
deep-sea soundings to which it could not be applied. But this 
is not a valid objection, for all the genera are represented by 
modern species, and these are sufficient for a basis of classifica- 
tions, and since the specific characters are based mainly on the 
structure of the valves, there will be no trouble with the fossil 
forms. The following synopsis of Petit’s system includes the 
higher divisions only. 
I. Bacillariaceæ coccochromatice. 
With numerous endochrome granules. 
A. Frustules concentrically constructed. One mother 
cell forming asexually a single auxospore. Melosi- 
res, etc. 
B. Frustules bilateral, one or two mother cells forming 
two auxospores, as far as known asexually. Fra- 
gilariex, etc. 
II. Bacillariacee placochromatice. 
With one or two large endochrome plates. 
A. One endochrome plate lying against the convex 
valve; one mother cell forming one auxospore 
asexually. Cocconeidex. 
B. A single endochrome plate extending diagonally 
across the cell cavity, or lying next the girdle. Two 
auxospores formed from two mother cells, with or 
without conjugation. Nitzschiex. Amphorex, Cym- 
belles, ete. 
C. Two endochrome plates lying next the two valves. 
Two mother cells forming two auxospores by con- 
jugation. Eunotiex, Synedricx, Surirayex. 
D. Two endochrome plates lying next the two girdle 
bands; two mother cells forming two auxospores 
without conjugation. Amphiplewree, Naviculex, etc. 
