Mr. C, Mereschkowsky on Californian Diatoms. 477 
0°120-0°150 mm.), length of the body 0°028-0°042 mm., 
breadth of the valve 0°003-0 008 mm. 
This I consider now as the type species, the var. genuina, 
which I formerly thought to represent the type, having proved 
to be much rarer. 
It is a very delicate, almost membranaceous diatom ; the 
rostra are very fine, flexible, usually slightly inflated at the 
apices; While moving, the frustule is constantly turning 
around its longitudinal axis in the same way as in Cylindro- 
theca. ‘Vhe girdle-face does not differ from the valve aspect 
as it does in WV. gracilis, N. biplavata, NV. acicularis, N. lon- 
gissima, &e., the rostra being cylindrical, not flattened as in 
the above-mentioned species, and the inflated lanceolate part 
ot the frustule or “ body ” being as distinctly separated from 
the rostra in the valve- as in the girdle-face (compare a and 
6 in fig. 2). A form with straight rostra can be distin- 
guished from the type species as 
Forma directa. (PI. LV. figs. 4,5.) San Pedro, very common. 
[M.] 
General length 0:098-0°166 mm., length of the body 
0:020-0:038 mm., breadth of the valve 0:003-0°0075 mm. 
Although I have occasionally seen such forms in other 
localities (Black Sea, Villetranche), it is only in California 
(San Pedro) that I have met with it in great abundance, 
always, however, in company with the type. I do not think 
it advisable to consider such forms as a variety unless they 
are found in unmixed assemblages. 
122. Nitzschiella tenuirostris, var. parva, Mer. (PI. IV. 
figs. 8-10.) San Pedro, not rare. [M.] 
Differs from the type only by its smaller siz2, being about 
0:055 mm. in length. 
In my paper on the Diatoms of the Black Sea I did not 
separate this smaller form from the larger ones which I now 
consider as the type species, and I would not do it even now, 
preferring to consider the small forms merely as reduced 
individuals of the type; but since then I have examined a 
gathering from ‘Theodosia (Crimea) in which this small form 
is extremely abundant and in a perfectly pure condition, all 
the individuals having the same aspect and never attaining 
the size of the type species. This fact compels me (L must 
confess against my will) to regard it as a distinct variety. 
The same form is not rare im San Pedro, although mixed 
with the larger type species. 
