KONGL. SV. VET. AKAD^MIENS HANDLINGAR. BAND 26. N:0 2. 121 



This species is very variable as to size and number of the strige, so that I cannot admit 

 the separation of Bhoic. compactum and Rhoic. oceanieum Per. According to Peragallo Gyr. com- 

 pactum has 20 to 24 transverse and closer longitudinal striae, but I have not seen such closely 

 striate forms, which are said to be frequent. Most specimens I have seen agree with Rhoic. ocea- 

 nieum. The fact that the valves of the same frustule are dissimilar, discovered by Perasallo, is 

 of great interest. The striation of the valves is somewhat dissimilar. On specimens from La 

 Rochelle 1 counted on the lower valve 19 transv. and 24 longit. striae in 0,oi mm. and on the 

 upper valve 16 transv. and 23 longit. striae in 0,oi mm. 



30. G. (Rhoicosigma) luediterranenm Cl. (1877). — V. narrow, with acute ends. L. 0,i8 

 to 0,24; B. 0,022 to 0,0025 mm. Median line diagonal in the middle and then marginal. T.S. : L.S. 

 *V25> '726. '727 in 0,01 mm. — Rhoic. medit. Cl. T. R. M. S. 1877 p. 182. N. R. D. p. 6 PI. If. 9. 

 Per. IX f. 29 to 32. 



Marine: Balearic Islands! Adriatic! Sumatra (Deby Coll.)! Java! 



Var. calcarea Brun (1891). — V. smaller, with less acute ends and less asymmetrical median 

 line. T.S. : L.S. 20.5/^^ in 0,01 mm. — Per. p. 32 PL IX f. 28. 



Marine: Japan, fossil (Per.). 



Var. chinensis Cl. — L. 0,i4 ; B. 0,o]8 mm. Median line as in the type. T.S. : L.S. ^Vao 

 in 0,01 mm. 



Marine: China (Deby Coll.)! 



31. G. (Rhoicosigma) robustum Grun. (1880). — V. narrow lanceolate, sigmoid, gradually 

 tapering from the middle to the acute ends. L. 0,27 to 0,6; B. 0,04 to 0,06 mm. Median line 

 sigmoid, excentric. T.S. : L.S. "/12) '^/la. "As ii^ 0,0 1 mm. — Pleur. (Rhoic?) roiustum Grun. 

 A. D. p. 58. Rhoic. robustum Per. X f. 2, 3. 



Marine: Mediterranean Sea! Java! Singapore! Samoa! Galapagos Islands! Campeachy Bay! 

 Var. inflexa Per. (1891). — Valve more narrow and sigmoid. — Per. p. 34 PI. X f. 4. 

 Marine: Mediterranean Sea! 



Frustulia Ac. (1824). 



Both valves similar. Central nodule small, indistinct or elongated. Median line enclosed 

 between two siliceous ribs. Terminal nodules small, sometimes elongated; terminal fissures not 

 distinct. No axial or central area. Structure: puncta arranged in transverse and longitudinal 

 striae. Connecting zone simple. Cell contents : two endochrome-plates along the interior wall of 

 the connecting zone, in the middle of the valve separated a from the wall by hemispherical plasma- 

 masses. On division of the plates, they do not move in the cells ; the fission begins at the ends of 

 the plate (Pfitzer Bau u. Entw. p. 58). On conjugating, two frustules form by their cell-contents 

 a mass, which is transformed into two cylindrical bodies, with obtuse, rounded ends, coarsely 

 transversely costate, parallel to the empty valves. The ends of these bodies form, later on, caps, 

 which are thrown oif. The bodies become conical, and gradually the valves are developed, one after 

 the other. When full-grown they are twice as long as the mother-cells (Pfitzer Bau u. Entw. p. 58). 



From the description of the cell-contents and the process of conjugation it seems that Fru- 

 stulia and Navicula differ considerably in these respects. 



Some species of Frustulia (of the group of F. rhomhoides) are, as far as regards the valve, 

 nearly akin to the Naviculce orthostichw and, on the other hand to Amphipleura. The central nodule, 

 usually small, becomes in same forms {N. rhomhoides var. amphipleuroides) united to the strong 

 siliceous strings, which enclose the median line, as in Amphipleura, bat the central nodule is in 



K. Sv. Vet. Akademiens Handlingar. Bd 26. N:o 2. 16* 



