KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 26. N:0 2. 29 



Marine; G-uUtnarsfjord, Sweden! Atlantic coast of N. America (Lewis.). 



This species, which I have found at Fiskebackskil (West-coast of Sweden) mong Zostera, is 

 scarcely silicious, the frustules disappearing almost completely, if burnt on a glasscover. 



This very interesting species, according to Lewis lives in colonies, and forms curved iila- 

 ments of 12 or more frustules. He figures such a colony in top-view, which shows that the 

 frustules keep in contact by means of the wings. 



Other species, which perhaps belong to Amphoropsis are Amphipr.? paradoxa Grbv. (Edinb. 

 N. Ph. J. XVIII p. 41 f. 21, 1863) remarkable for its coarsely moniliform strise, and Amph. 

 Thwaitesiana Grev. (1. c. p. 183 f. 9) notably distinguished by its strongly marked wings, which are 

 striate, other parts of the valve not showing any striation. 



Dictyoneis Cl. (1890). 



Valve elonga,ted, panduriform or lanceolate. Median line straight with the terminal fissures 

 usually in contrary directions. Valve with double structure; the upper stratum finely punctate, 

 with puncta disposed in decussating rows; the interior stratum irregularly reticulate with rounded 

 cellules. Marginal cellules frequently larger than the others, forming a row of false loculi. Con- 

 necting zone not complex. 



The first known species of this genus is Navicula marginata Lewis. The larger marginal 

 cellules of several species give to the valves some appearance of Mastogloia, in which genus seve- 

 ral forms of Dictyoneis have by various authors been placed. In the year 1877 Gbtjnow expresses 

 the opinion that the margijial cellules seem not to belong to a separate plate as in Mastogloia, 

 but to the valve itself and proposes a new genus for Mast.? reticulata and the allied species. In 

 the year 1890 I proposed (Diatomiste I p. 14) for this genus the name Dictyoneis, founded on the 

 peculiar structure of the valve. Unfertunately I have in that paper committed an error, as I then 

 supposed the stratum with coarse reticulations to be the upper instead of the lower. 



The median line of most species of Dictyoneis is straight and ends in terminal fissures, turned 

 in contrary directions. The central pores are moderately distant, except in D. naviculoides, where 

 they are very approximate. On both sides of the median line are narrow structureless zones, which 

 are more silicious than the other parts of the valve and are united to the central nodule. 



The genus Dictyoneis is well distinguished from all the other naviculoid genera. Its syste- 

 matical place is doubtful, as there are no intermediate forms connecting it with other diatoms. 

 The structure of the outer stratum of the valve as well as the terminal fissures point perhaps 

 to some relation to Pleurosigma. 



The species are all marine and belong to warmer seas. Fossil forms occur at Oamaru and in 

 Hungary. 



This genus comprises forms, which are so closely connected, that the distinction of well 

 defined species is very difficult, as is frequently the case with truly natural groups. Such charac- 

 teristics as the outline of the valve, the coarseness of the reticulation etc. are very variable. 



Artificial hey. 



^ j Marginal cellules larger than the others ... . .3. 



\ — — not — — . . 2. 



„ J Valves pandnriform D. jamaicensis Grev. 



\ — lanceolate D. naviculacea Cl. 



Valve not constricted ....£). Thumii Cl 



3. ■{ — slightly constricted . . . .... 4. 



— strongly . . . . .... . ... 6. 



