142 v. T. CLEVK, SYNOPSIS OF THE KAVICULOID IJIATOMS. 



4. Libclluv. Zone complex or with loiigitudiiial divi.sioiis. Central nodule dilated into a 

 stauros, ar not dilated. 



5. Microstigma. Zoue simple. Central nodule not transversely dilated. 



This division of tbe 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 gronps, viz. tho.se with transversely dilated central nodnles in Stauroneis, and those withoiit 

 snch dilated central iiodnles in Microstigma; at the same time indicating as far as can yet be 

 made ont to which of the five gTonps named abovp each species belongs. 



The genus Stauroneis (Ehr. 184;i) has always been regarded as distinguished from Xavicula 

 bj^ 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 Trachvneis, 

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

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

 Microstigmaticffi. 



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

 the structxxre 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 stritt are radiate at the ends. This is the case also 

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

 trän sition s. 



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

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



The fresh Avater species of Sc/ihostauroii are closely allied to those of Pleurostauron, but 

 I am not sufficiently acip^ainted with the marine species of the former to be able to speak as to 

 their affinities. 



The division Libellus comprises forms with and withont stauros. They are all marine and 

 some of tliem 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 \\\ey cannot be specificall_v distinguished. there is no reason to 

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

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

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

 species sufficiently; but as it is tigured in Van Heurcics 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 stria' (A', auldaudica 

 and A\ Garkeana). Still closer allied are the forms of the section Oxyamphora among the asym- 

 metrical naviculoid diatoms. The same structure exists in these Amphor;B 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 Microstitjma comprises forms without stauros, and with a simple counecting 

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

 have enclosed in this di\ision several forms, classed in difierent genera by authors. as Scolio- 

 pleura tumida, and Rhoicomis Garkeana, the former haA'ing a sigmoid median line, the latter 

 arcuate fi-ustules. On the sigmoid median line alone no natural genus can be founded, forms with 



