KONGL. SV. VET. AKADEMIKNS IIANDLINGAR. HAND 26. Nto 2. 121 



This species is very variable as to size and number of the stria;, so tliat T cannot admit 

 the separation of Rhoic. compactum and lUioic. oceamcuni Per. According to Pkraqallo Gyr. roni- 

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

 striate forms, which are said to be freijuent. Most specimens T have seen agree with Bhoic. orea- 

 nicum. The fact that the valves of the same frustnle are dissimilar, discovered by Pekagallo, 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. stria; in 0,oi mm. and on the 

 upper valve 1<) transv. and 23 longit. strife in 0,oi mm. 



30. Gr. (Rhoicosigraa) raediterraneuiii Cl. (1877). — V. narrow, with acute ends. L. 0,i8 

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

 18/25, ^726- 'V27 in 0,01 mm. — Rhoic. medit. Cl. T. R. M. S. 1877 p. 182. N. R. D. p. G PI. I f. 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. 2"-/27 in 0,oi mm. — Per. p. 32 PL IX f. 28. 



Marine: Japan, fossil (Per). 



Var. chinensis Cl. — L. 0,i4; B. 0,oi8 mm. Median line as in the type. T.S. : L.S. 23/^„ 

 in 0,01 mm. 



Marine: China (Deby Coll.)! 



31. 0. (Rhoicosigma) robustiim Grun. (1880). — V. narrow lanceolate, sigmoid, gradually 

 tapering from the middle to the acute ends. L. 0,2 7 to 0,6; B. 0,04 to 0,o6 mm. Median line 

 sigmoid, excentric. T.S. : L.S. "/i2' ''/is' 'Vis in 0,(ii mm. — Pleur. (Rhoic?) rohusfum Grun. 

 A. I), 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 Ao. (1824). 



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

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

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

 striffi. Connecting zone simple. Cellcontents: 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-co)itents 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 NariculfP orthostichrf and, on the other hand to AmphipJciiia. The central nodule, 

 usually small, becomes in same forms {N. rhomJtoides var. (impliipleurnides) united to the strong 

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



K. Sv. Vet. Akademiens llandlinj'ar. Bd 'i(j. N;o 2. 1(5 



