478 Mr. C. Mereschkowsky on Californian Diatoms. 
A still smaller form, 0°035-040 mm. in length, with very 
short rostra, can be distinguished as 
Forma minutissima. (PI. IV. fig. 11.) San Pedro, rather 
rare. [M.] 
The forma semictreularts and varietas arcus, Mer. (the 
latter attaining 0°364 mm. in length), belonging to the same 
group, are mentioned in my paper on the Diatoms of the 
Black Sea. I have not seen them in the Pacific. 
125. Nitzschiella tenuirostris, var. hamulifera, Mer. (N. tens7- 
rostris, var, genuina, Mer. Diat. Black Sea). (PI. LV. 
figs. 6, 7.) San Pedro, rare. [M.] 
Differs iy the very thin elongated rostra, which are straight, 
becoming curved and spirally twisted at the ie only, 
where they form a hook. General length 0°140-0°194 mm., 
length of the body 0-045 mm., breadth of the valve 0°006 mm. 
This variety is very frequent in some localities of the 
Black Sea. I have found it also in the Mediterranean (Ville- 
franche). The hooks are not always so distinct as repre- 
sented in fig. 6. 
124. Nitzschiella gracilis, Mer. (PI. IV. figs. 12,13.) San 
Pedro, rare. [M.] 
This is quite a distinct species, differing from WN. tenu7- 
rostris by the more robust frustule, the rostra being broader, 
straight, curved in the same direction at the ends only ; they 
are flattened, especially at the apices, which makes them 
appear Greaden and more hyaline at the girdle-face. Hndo- 
chrome composed of two plates. While moving, the frustul; 
does not turn around its longitudinal axis. Length 0°126- 
0°165 muwn., attaining 0°-245 mm., breadth of the valve 0:0052- 
0-008 mm, 
125. Nitzschiella gracilis, var. reversa, Mer. (PL IV. fig. 14.) 
San Pedro, very common. [M.] 
Rostra curved in opposite directions, General length 
0-1-0°175 mm., length of the body about 0°05 mm.. breadth 
of the valve 0°008-0-009 mm. Very common in San Pedro, 
Ww here the type species 1s very rare ; the opposite is tlie case 
in Villefranche (Mediterranean) and the Black Sea. Var. 
reversa 18 also frequent in fresh water, California (Los 
Angeles). 
The following table will facilitate the distinction between 
