142 p. T. CLEVK, SYNOPSIS OF THE NAVICULOID DIATOMS. 



4. Libellu.^. Zone complex or with longitudinal divisions. Central nodule dilated into a 

 stauros, or not dilated. 



5. Microstigma. Zone simple. Centi'al nodule not transversely dilated. 



This division of the whole group cannot be completely carried out at present, as the con- 

 necting zones of many species have not been observed. I consequently class all the forms now in 

 two groups, viz. those with transversely dilated central nodules in Stauroneis, and those without 

 such dilated central nodules in Microstigma; at the same time indicating as far as can yet be 

 made out to which of the five groups named above each species belongs. 



The genus Stauroneis (Ehr. 1843) has always been regarded as distinguished from Navicula 

 by the transversely dilated central nodule; but the difference between a dilatation of the nodule 

 itself, or stauros, and a more transverse extension of the central area, or fascia, has not hitherto 

 been strictly carried out, and several forms with transverse areas only have been placed in 

 Stauroneis. These I now remove to Pinnularia or Navicula. 



Still, if we include in Stauroneis all the naviculoid forms which have transversely dilated 

 central nodules, we shall not have a natural genus, as it will comprise species of Trachyneis, 

 Mastoneis, Pseudoamphiprora, and other groups. All these I also remove to their respective groups 

 and confine Stauroneis to such forms as possess the structure described above as belonging to the 

 Microstigmaticse. 



Stauroneis has affinities with Amphora, particulary with the group Psammamphora, in which 

 the structure is identical, and in which occur species with and without a stauros, which may be 

 regarded as asymmetrical forms of Stauroneis and Microstigma. 



In all the true species of Stauroneis the striae are radiate at the ends. This is the case also 

 in the group Pleurostauron, which is nearly related to Stauroneis and passes over into it by gradual 

 transitions. 



Many species of both these groups are met with in fresh water in all countries, arctic or 

 tropical, a few inhabit brackish waters, but there is scarcely any undoiibted marine species. 



The fresh water species of Schizostauron are closely allied to those of Pleurostauron, but 

 I am not sufficiently acquainted with the marine species of the former to be able to speak as to 

 their affinities. 



The division Lihellus comprises forms with and without stauros. They are all marine and 

 some of them live enclosed in gelatinous tubes, for which reason they have been classed in the 

 very unnatural genus Schizonema. As there are free forms, so closely connected with those 

 living in gelatinous tubes, that they cannot be specifically distinguished, there is no reason to 

 retain the genus Schizonema. As early as 1873 I proposed the generic name Libellus for Naviculse 

 with complex zone, but this view was not accepted until recently by De Toni, who placed in this 

 genus N. aponina. Whether this be admissible or not I cannot say, as I have not examined this 

 species sufficiently; but as it is figured in Yan Heurcks Synopsis as having subsidiary longitudinal 

 lines it may belong to Caloneis. 



The species of Libellus are no doubt closely connected with those of the division Microstigma, 

 in which are some forms the zone of which has longitudinal rows of short strise (iV. auldandica 

 and N. Garheana). Still closer allied are the forms of the section Oxyamphora among the asym- 

 metrical naviculoid diatoms. The same strvicture exists in these Amphorte as in Libellus; the 

 zone is similar and there are in Oxyamphora species with, and without stauros, exactly as in 

 Libellus. Most forms of Libellus have the terminations of the median line at some distance from 

 the ends of the valve. 



The division Microstigma comprises forms without stauros, and with a simple connecting 

 zone, which however, as stated above, has in some species longitudinal rows of short strife. I 

 have enclosed in this division several forms, classed in different genera by authors, as Scolio- 

 pleura tumida, and Ehoiconeis Garheana, the former having a sigmoid median line, the latter 

 arcnate frustules. ' On the sigmoid median line alone no natural genus can be founded, forms with 



