680 Transactions of the Society. 



Pleurosigma. 



Tleurosigma attenuafum, var. Caspia Grunow. — Somewhat 

 smaller, narrower, and less sigmoid than the type species, apices 

 less acute. Strise the same as in P. attenuatum and P. Hippo- 

 campus. 



Not uncommon in Baku Harbour. 



Between P. attenuatum and P. Hippocampus there exists only 

 the unimportant distinction, a difference in the outline of the valve. 

 The Caspian form is distinguished from both by being narrow, less 

 sigmoid, and by its more obtuse apices. The structure of these 

 alhed forms imder higli powers appears very similar ; between the 

 strongly-marked hues of beads faint outlines of other beads may be 

 seen. Whether these delicate puncta belong to a second valve or 

 are an optical delusion must remain for the present undecided ; 

 it is certain, however, that the valves of Pleurosigma are com- 

 posed of two layers, which separate when acted upon by long 

 boiling in acids. [I have observed this in P. angulatum.'] 



[The faint markings here alluded to have been seen by other 

 observers. It is most probable that the valves of Pleurosigma have 

 a similar structure to many other diatoms in possessing what I call 

 " secondary valves," which in some genera are like, and in others 

 unlike, the primary valve. Schmidt calls them EegenerationshuUe, 

 — F. K.] 



P. elongatum, var. gracile Grunow. — Narrower and less sharply 

 pointed than Smith's figure. Striae somewhat stronger. Obhque 

 striae, 17-18 in -01 mm.; transverse, 19 in '01 mm. Length, 

 •13-- 32 mm. ; breadth, • 024- • 025 mm. 



Common in Baku Harbour. 



SCHIZONEMA. 



ScMzonema {minutum Kg. var.?) Caspium Grunow. — Sch. 

 miuutissimum, filamentis brevissimis, subsimplicibus vel parce 

 dichotome ramosis, inferne irregulariter transverse rugosis, superne 

 laevibus, hyalinis, apicem versus vix conspicuis. Frustula inclusa 

 irregulariter hbera disposita, valvis lanceolatis, obtusiusculis, • 035- 

 •052 mm. longis, •007- '008 mm. latis, nodulo centrali parvo 

 oblongo, striis parallelibus 12 in • 01 mm., in media parte parum 

 distantioribus, subradiantibus, in area minuta nodulum centralem 

 ambiente deficientibus. 



On Cladoplwra in Baku Harbour common. 



In 8. minutum, Kg. Bac, pi, 23, fig. 5, the frustules occur 

 in straight lines, in which it resembles 8. humile Kg. ; but in tlie 

 latter species they principally occur in the chief filament (Haupt- 

 schlauch). In the Caspian form this is never the case. 



