KONGL. SV. VET. AKADEMIENS HANDL1N6AK. BAND. 27. N:0 3. 165 



Frastules with an interiör skeleton Campyloneis Gbun. 



3. •{ — — a marginal rim 4. 



— without steleton or rim 5. 



. I Upper valve costate, costae alternating witli double rows of punota Pleuroneis Cl. 



' \ — — pnnctate-striate Cocconeis Cl. 



j Lower valve with cost» alternating with double rows of puncta .... Ächnanthes Boey 



' ' • I — — ecostate 6. 



„ I Upper valvfe costate 7. 



I — — not costate ' 8. 



_ I Axial area of tlie upper valve broad or laneeolate Heteroneis Cl. 



' \ — — — — — narrow or a rib 10. 



Valve broadly elliptical Eucocconeis Cl. 



narrow, linear or laneeolate 9. 



9, 

 10, 



I Vah 



( 



\ 



( Structure: finely punctate striae 'Microneis Cl. 



\ — coarsely punctate striie Achnanthidium Cl. 



Valve elliptical Disconeis Cl. 



— laneeolate or rhomboid Acfinoneis Cl. 



Rhoicosphenia Grun. (1860). 



Valves dissimilar, strongly asymmetrical to the transverse axis, clavate. Upper valve 

 withoitt central nodiile and median line, but with an axial area. Lower valve with central nodule 

 and longitndinal line. Structure of both valves : transverse, finely punctate striee. Frustule usually 

 stipitate, cuneate, with short diaphragms at the ends. Connecting zone not complex. 



Oell-contents a single chromatophore-plate along one of the interiör walls of the zone and 

 both valves, with the opening along the other wall of the zone, with slight sinuses at the base 

 and the ends, and also towards the ventral nodule; but there is no deep fissure below the median 

 line. Division of the plate as in Gomphonema (Pfitzek, Bau u. Entw. p. 91). In conjugating 

 the cells behave as Gomphonema, but when the aiTxopores attain to the length of the mother-cells 

 they become enclosed in a silicious, transversely costate, membrane, inside which the primordial 

 cells originate (Thwaites). 



The species which for long was the only one in tliis genus was in 1833 named by Kutzing 

 Gomphonema minutissimum, or G. curvahwi, and was first (1860) by Grunow removed as RJioico- 

 sphenia curvatum to the family Achnanthew. Notwithstanding the dissitnilarity of the valves 

 Mhoicosx)henia bears a close resemblance to Gomjihonema both in exteriör and interiör respects. 

 There is also some affinity to the Tahellariecb, especially in the diaphragms at the ends of the 

 frustule. 



1. R. curvata KtJTZ (1833). — V. clavate, with rounded, obtuse upper end, and more 

 narrow, attenuated, obtuse base. L. 0,oi5 to 0,025; B. 0,ao3 to 0, 045 mm. Upper V. with narrow, 

 centrally placed axial area, and parallel strise, about 16 in 0,oi mm. Lower V. with narrow 

 or indistinct axial, and small elongated, central area. Median line with somewhat distant 

 central pores, and about 15 strise in 0, 01 mm., radiate throughout and stronger in the middle. — 

 GompJionema minutissimum Ktixz. Dec. N:o 76 (1833) according to Lägst. G. curvatum Kutz. 

 Linneea X p. 567 Pl. XVI f. 51 (1833), according to Lägst. W. Sm. B. D. p. 81 Pl. XXIX 

 f. 245, 246. EhoicospJienia curvata V. H. Syn. p. 127 Pl. XXVI f. 1 to 3. Gompli. marimim 

 W. Sm. b. D. i p. 81 Pl. XXIX f. 246. 



Fresh and brackish water: Arctic America! Greenland! Spitsbergen! Finmark! Cape Desch- 

 neff! Behring Island! Europé generally, Baltic and Caspian seas! Atlantic coasts of Europé and 

 America! Cape Good Hope (Grun.), New Zealand (Grun.), S:t Pauls Island! Honduras (Jan. E,abh.)_ 



Var. major Cl. — L. 0,07; B. O.oos mm. Strite 9 in 0, 01 mm. 



Fresh water: Pitt River, Oregon! 



