KONOL. SV. VET. AKADEMIENS IIANDLINOAR. BAND. 27. .\:() 3. 191 



Achnanthidium (KCttz. 1844) HEin. 1S6.3. 



Outline more or less linear, rarely elliptical. Valves dissimilar. Upper valve without cen- 

 tral nodule and median line, but with narrow, frequently asymmetrical axial area, not costate 

 (except A. groenla.ndica). Structure transverse rows of puncta (except A. groenlandica). Lower 

 valve with straight, central median line. Central nodule usually stauroid. Central area a trans- 

 verse fascia. Structure: transverse rows of puncta. Frustule genuflexed, without rim. Connecting 

 zone usually with longitudinal rows of short striaj. 



Cell-contents (of A. lanceolata) a single chromatophore-plate along the inside of the upper 

 valve or (as in A. breripes) two plates, deeply sinuose in the middle along the inside of the 

 connecting zone. In conjugation one cell {A. brevipes) gives origin according to Luders to one 

 anxospore (Pfitz. Ban nnd Entw. p. 85). 



This group comprises most of the stipitate species of Aclmantltcs of authors. They agree 

 in outline with the true Achnanthes, which have an entirely different structure. Whether the 

 genus formed by me is natural is somewhat doubtful, the cell-contents of A. lanceolata being very 

 similar to those of Cocconeis, and those of A. brevipes agreeing with those of Naviciila. On the 

 other hand A. (/roeiilandica differs in the costate upper valve and is remarkable for the rudimen- 

 tary diaphragms at the ends of the frustule, which point to some affinity with the Tabellaricfi 

 (Efifopt/la). The exact position of this group in a natural system is impossible to define at present. 



Artificial key '). 



IJIargiii of tlie upper valve with a horseshoe-shaped mark A. lanceolata Br:^b. 

 (A. Peragalli Br^b. a. Heeib.). 

 — — — — without — — — 2. 



Q J Margin frenulated A. crenulata Grun. [A. brevipes var. snbcrenulata). 



"' \ — not — 3. 



j Upper valve with transverse costse A. groenlandica Cl. 



1 — — — — rows of puncta 4. 



J Axial area of the upper valve central 5. 



1 — — — — — excentric G. 



I Valve linear A. mesogongyla Grun. 



\ — lanceolate A. niargaritarum Cl. 



J Valve centrally gibbous A. inflata Grun. 



\ — — not — 7. 



^ j Valve small and constricted A. coarctata Be:6b. 



\ — — — elliptical or large, and constricted, or not A. brevipes Ag. 



r. A. niargaritarum Cl. (1893). — V. not arcuate, lanceolate, with slightly protracted 

 ends. L. 0,0.5 to O.ot;; B. 0,02 mm. Upper V. with central, narrow axial area. Stria 10 (middle) 

 to 13 (ends) in 0,oi mm. very slightly radiate, composed of distinct puncta, arranged in straight, 

 longitudinal rows, If) in 0,oi mm. Lower V. with straight median line, indistinct, axial and small, 

 orbicular central area. Stripe 10 (middle) to 15 (ends) in 0,01 mm. composed of puncta, arranged 

 in straight longitudinal rows, 13 in 0,oi mm. — Diatomiste II p. 57 PI. Ill f. 9, 10. 



Marine: Pearl Islands (near Java?)! 



This remarkable species is not akin to any other. The upper valve has some resemblance 

 of Tiluiplioncis amphiceros. 



2. A. lanceolata Br:^b. (1849). — Narrow, elliptic-lanceolate to broadly elliptical, with 

 rounded, obtuse, rarely rostrate, ends. L. 0,oi7 to 0,o.3r); B. 0,oo.5 to 0,oo8 mm. Upper V. with 



') The incompletely known Ach. heccata Leud. Fortm., A. agglutinatn Grun. and Cocconeis perpusilla Pant, not 

 included. 



