KONGl.. vSV. VKT. AKADEMIPIN.S 11 ANDLINGA K. HAN1>. 27. N:0 3. Hi') 



[ Frustules with an interior skeleton Campyloneis Grdn. 



3. ! — — a marginal rim 4. 



I — without skeleton or rim 5. 



, I Uijper valve costate, cost<e alternating with doulile rows ol' ])iiii(ta Pleuronein Cl. 



' I ^ — punctate-striate Cocconcis (!l. 



) Lower valve with costa? alternating with double rows of puncta .... Achnanthes Hory 

 ' ' ) — — ecostate 6. 



I Upi)er valve costate 7. 



) — — not costate 8. 



I Axial area ot the upper valve broad or lanceolate Heteroneis Cl. 



\ — — — — — narrow or a rib 10. 



„ j Valve broadly elliptical Encocconeis Cl. 



I — narrow, linear or lanceolate 9. 



Structure: finely punctate striae Microneis Cl. 



— coarsely punctate strife Achnanthidium Cl. 



9 



.. I Valve elliptical Disconeis Cl. 



1 — lanceolate or rhomboid Acfivoneis Cl. 



Rhoicosphenia Grun. (1860). 



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

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

 and longitudinal line. Structure of both valves : transverse, finely punctate striffi. Frustule usually 

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



Cell-contents a single chromatophore-plate along one of the interior 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 fissvtre below the median 

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

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

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

 cells originate (Thwaites). 



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

 Gomphonema tninutissimum, or G. curvatum, and was first (1860) by Grunow removed as Rhoico- 

 sphenia curvutum to the family Achnanthea?. Notwithstanding the dissimilarity of the valves 

 Rhoicosphenia bears a close resemblance to Gomphonema both in exterior and interior respects. 

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

 frustule. 



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

 narrow, attenuated, obtuse base. L. 0,oi5 to 0,025; B. 0,oo.3 to 0,o45 mm. Upper V. with narrow, 

 centrally placed axial area, and parallel stricB, 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 strite in 0,oi mm., radiate throughout and stronger in the middle. — 

 Gomphonema minutissimum Ktlxz. Dec. N:o 76 (1833) according to Lagst. G. curvatum Kutz. 

 Linntea X p. 5()7 PI. XVI f. 51 (1833), according to Lagst. W. Sm. B. D. p. 81 PL XXIX 

 f. 245, 246. Rhoicosphenia currafa V. H. Syn p. 127 PI. XXVI f. 1 to 3. Gomph. marinum 

 W. Sm. B. D. I p. 81 PI. XXIX f. 246. 



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

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

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



Var. major Cl. — L. 0,07; B. O.ods mm. Strise 9 in 0,oi mm. 



Fresh water: Pitt River, Oregon! 



