190 p. T. CLEVE, SYNOPSIS OF THE NAVICULOID DIATOMS. 



Var. costalonga Beun a. 'S.tsiB. (1893). — Valve very slightly curved. Longitudinal lines 

 more distinct. — 1. c. f. 13. 



Var. major Brun a. H^rib. (1893). — L. 0,2 3 mm. — 1. c. f. 12. 



A very remarkable species. It does not belong to Gromphoneis as the strise are composed of 

 simple rows of puncta. 



Tracliyneis Cl. N. Gen. 



Valve naviculoid, more or less elongated, frequently asymmetrical on both sides of the longi- 

 tudinal axis. Valve (often) vpith an interior coarsely dotted stratum, a median stratum of more 

 or less transverse flexuose strong costse, anastomosing, where they bend towards each other and 

 thus forming a network of diamond-shaped or rectangular alveoli, and an exterior stratum with 

 very fine puncta forming longitudinal, sometimes slightly oblique, fine strise. — Cell-contents: T. aspera 

 V. genuina has two chromatophore-plates along the connecting zone. The margins of the plates 

 are strongly indented. 



The type of this genus, Nav. aspera Ehb. is very variable and widely distributed. The 

 peculiar structure of this species makes it necessary to form for it and the allied species a 

 separate genus. Under a low power the valve seems to be covered with elongated or rhomboid 

 puncta, more or less distinctly disposed in quincunx, these puncta being the diamond-shaped alveoli 

 formed by the anastomoses of the costse. The fine puncta of the exterior stratum are visible only 

 with difficulty, and, under certain illumination, give to the elongated puncta the appearance of 

 being transversely lineate (See Van Heurck Suppl. PI. B f. 26). The interior stratum of the valve 

 may sometimes be separated, as is shewn by the fig. 23 PI. XL VIII of A. S. Atl. 



I suppose that the division Lineolatse of Navicula may be regarded as having the nearest 

 relations to Trachyneis, especially the forms of that group, which have divided striae. A struc- 

 ture somewhat similar to that of Trachyneis is to be found in Mastoneis and Dictyoneis. All the 

 species of this genus are marine. They occur in all seas, arctic as well as tropical. 



Most forms of Trachyneis aro so closely connected and s.ubject to transitions as to make 

 the distinction of well founded species very difficult. The variability of some species is considerable. 

 In certain species there is a remarkable tendency to asymmetry, some varieties having a very ex- 

 centric median line, or asymmetrical axial area; but such forms, for which the genus Alloioneis 

 was created, are so closely connected with other, nearly, or perfectly, symmetrical forms, that no 

 specific distinction can be founded on this character. The arrangement of the alveoli in oblique, 

 or straight longitudinal rows, seems to afPord some distinction ; but this characteristic is also subject 

 to great variation, and in some forms we find that the rows are oblique near the central area, 

 and longitudinal elsewhere. 



Artificial key. 



^ f Central area small . . ■ . T. velata, A. S, 



' \ — — large .... 2. 



I Central area a transverse, outwards dilated fascia 3. 



' \ — — rounded 4. 



M' 



Stauros not reacMng to the margin T. aspera Ehb. 



reaching — — .... T. tumidula Grun. 



I Alveoli in decussating rows 5. 



■ \ — — longitudinal — .... T. Johnsoniana Geev. 



I All alveoli in decussating rows 6. 



\ Alveoli at the margin in decussating, in the middle in straight rows ... T. Brunii Cl. 



