KONGL. SV. VET. AKADEMIENS HANDLINGAK. BAND 26. N:0 2. 7 



as for instance Navicula Raeana and Nav. Yarrensis, Nav. tumida and Nav. xjlicata, Navicula 

 Sif/nia and Nav. supcrimposita, Cocconeis heteropleura and C. 2)ellucida. 



The direction of tlie ferminal fissure-9 in most Navicnlre is the same in both extremities, 

 but tbere are forms in wliich tliey are disposed in contrary directions. The latter characteristic 

 on which Grunow was inclined to fonnd a separate genus, Pseudopleurosigma, occtirs in widely 

 different groups, and it wonld not be iu accordance with a natnral system to class in one genus 

 all forms with the terminal fissures in contrary directions. Such forms are freqviently allied to 

 others "with the fissures in the same direction. On the other hand as a specific characteristic it 

 seems to be of verv great value. 



Areas, or structureless parts of the valve, occur usually along the median line and around 

 the central nodule. I call these areas axial and central areas. There are also, sometimes, lateral 

 areas, or blank spaces on both sides of the median line, separated from the latter by a striated 

 portion. The form, the j)resence, or absence, of such areas is in mo.st cases a valuable specific 

 characteristic, and, frequently, a useful distinction in groups of allied forms. 



Longifudinal lines. In a great number of forms there occur lines, parallel to the median 

 line, in some cases dilated into longifudinal bands or lateral areas, limiting an interiör elevated 

 or depressed portion. Although such longifudinal lines or lateral areas occur in certain groups of 

 forms most of which are devoid of them, as in the Nariculce lineolata, and in the whole group of the 

 NaviculcB lyratee, they are nevertheless in my opinion of great .systematic importance. In the Naviculee 

 hjratce the longifudinal lines or lateral areas are expansions of the central nodule, corresponding with 

 the horns') of Biploneis. In other cases they are merely non-striate portions of the valve, but in 

 AmpJiiprora, Diploneis, Caloueis they are of a different nature. In Amphijirora they mark the limit 

 between the elevated interiör and the fiatter exteriör parts, which frequently have a difFerent struc- 

 ture. They appear to be of the same nature in Caloneis. In Biploneis they limit a depressed 

 portion, and the same is the case in several groups of Amphora, in which they are freqently so 

 placed as to be distinctly seen only in favourable positions of the valve. In several cases the 

 longifudinal lines are double. In Diploneis I call the space between the median line and the in- 

 teriör longifudinal line the fitrrow, and the space between two longifudinal lines the lunida. The 

 presence or absence of longifudinal lines appears to me to be of very great importance in the con- 

 strucfion of genera. 



Structnre of the valve. The valves of the naviculoid diatoms vary greatly in structure. In 

 most cases the valves are decorated with small or large puncta, disposed in difterent manners. 

 At present it seems to be generally admitted that these puncta are small cavities in the silicious 

 mäss of the valve. They are rarely scattered irregularly, but are usually arranged in transverse 

 rows, more or less radiate or parallel, called strice. The puncta are also so arranged as to form 

 straight, or nndulating, longifudinal rows, parallel to the median line, or decussating straight, or 

 cnrved, rows as in Pleurosigma and others. When the puncta are large they are called alvcoli 

 and have the appearance of more or less quadrate spaces, as in several forms of Diploneis and Am- 

 phora. The silicious mäss enclosing the alveoli in such cases has a reticulated appearance. I call 

 these siliceous walls of the alveoli, if forming continuous silicious ribs, eostce. The strong, trans- 

 verse silicious ribs of Dip)loneis Crabro are such cosfje. The alveoli or puncta are frequently united, 

 as in Pinnularia, Caloneis etc. In such cases the strite appear to be channels, communicating with 

 the interiör, in the larger Piiuudarice, by a larger fnramen or ocellus. The outside part of the 

 valve consists in several cases of a thin, mini^tely punctate stratum, iisually seen only with difiiculty. 

 In a variety of Diploneis sjilendida, called Dip. diplosticta, there is such a finely punctate stratum 

 outside the layer of alveoli. In the genus Dictyoneis it is also visible, the minute puncta forming 

 transverse, and at the same time obliquely decussating strife. This is also the case with the genus 

 Trachyneis, where the fine puncta form longifudinal or somewhat obliqiie lines. In the groiip Navi- 

 cidce lineolatfP the transverse .strite are crossed by fine lines, which appear to be formed by clo.sely 

 erowded alveoli, as transitions exist from stria; of this nature to striae composed of distinct 



