10 



Naviculse Lineolatse Cl. 



Valve elongated, rarely constricted, sigmoid or asymmetrical. Axial area usually indistinct 

 or iiarrow, rarely wide. Central area small or large. Structnre: radiate or parallel strife., finely 

 and transversely lineate, tlie lineation always closer than the strife. Connecting zone not com- 

 plex. — Nav. distans and N. directa have two cliroraatophore-plates along the connecting zone. 

 Their margins are not indented. N. {Scliigonema) mollis has the same arrangement of the cell- 

 contents. 



This section corresponds to the groups Badiosce, Refuscp. and Directce of Grunow and coni- 

 prises a very large number of forms, frequently nearly connected or passing into each other. 

 Their most characteristic feature is the lineation of the striiB. The latter are usually radiate in 

 the middle of the valve and divergent, parallel or convergent at the ends. In several forms the 

 stricE are parallel throughout. In those forms in which the median stiiiB are radiate, they are 

 also frequently alternately longer and ahorter. 



This group is connected by a few forms (iV. Placentula) with the Naviculse PunctatiS. As 

 a rnle the Lineolatse incline to a lanceolate or linear outline, while the PnnctatsE are elliptical. 

 The principal distinction between both groups is that the strise of the PunctatcB are composed of 

 distinct puncta, usually as widely apart as the strise, but in the Lineolatee the puncta are much 

 closer, the distance between the lineolte being less than the distance between the strise. In some 

 cases (for instance Nav. tusculd) the striai are crossed by longitudinal blank lines and have then 

 very much the appearance of the striation in the section Punctatte.. Such forms may in most 

 cases be distinguished from the Punctatfp^ by the terminal strife. being parallel or convergent. 



Among the asymmetrical diatoms there is a close relation between the Cymhellee and this 

 section, both having in many cases an exactly similar striation. Several intermediate forms exist. 

 For instance Nav. dicephala is nearly akin to Cymbella anglica, and the frustules of Gynibella 

 Ungeri are frequently so nearly symmetrical, that this species might be classed among the 

 Lineolatte. Among the Amphorce there is one species only, as far as I know, which seems to be 

 closely akin to the Lineolatse (Amphora Uibuensis). Passages between the perfectly symmetrical 

 and decidedly asymmetrical are oifered by Nav. mediterranea and Nav. toxonidea. 



The forms of this group, which have the strife crossed by several longitudinal blank lines, 

 suggest a connection with the genus Trachyneis. 



The species of the group LfevistriatfB are no doubt nearly connected with those of the 

 Lineolata3. They have the same general outline, and the strife are in similar directions, but 

 those of the former are apparently smooth. 



Artificial key. 



^ j Median line sigmoid N. Sigmo. Brun. 



' \ — — .straiglit 2. 



j, j Axial area distinct or broad 3. 



' \ — — very narrow or indiatincf 16. 



., J Axial area bilateral 4. 



\ — — unilateral N. scnlarifer Brun. ' 



. I Axial area asynnnetrieal N. irref/iilriris Pant. 



I — ■— symni(!trieal 5. 



Valve e(jnBtrieted in tlii; middle N. Chi Cl. 



not — — — G. 



{ 



' Be.tides: i\'. irregnlnrin Pant.. N. mqierha Ci,., N. trcin.iitans rar. iiicuilif örmin, rnr. nsymmetrira. N. spuria 

 var. anymnietrica, N. imjierfccta Cl., N. monodoti Brun, N. Arithracis ]5r. and Ou., N. valida Cj,.' 



