b P. T. CLEVE, SYNOPSIS OF THE NAVICULOID DIATOMS. 



But in otlier cases the form is subject to much change as, for instance, in Navicida mutica. The 

 shape of the ends is frequently not constant, as in the genus Neidmm, Xancnla viridula, and others. 

 In tlie same natural group of diatoms both the oiitline and the ends are frequently subject to much 

 variation. I therefore conclude that thongh the shape of the valve may be of service to some 

 extent as a specific distinction, it is of no real importance in limiting or defining genera. 



On the sigmoid flexure of the valves have been founded the genera Fleurosigma and Scolio- 

 2)leura. This characteristic is subject to great variation in Fleurosic/nia, some forms of -which are 

 almost straight. On the other hand "widely diflferent forms of other genera are sigmoid, as I\aricida 

 Eaeana, Navicula Sigma, Galonets staurnphora. I have occasionally seen a sigmoid valve of Frus- 

 tulia Lewisiana and one of Navicula cincta var. Heufleri. The sigmoid flexure of the valve can 

 only be regarded as a specific characteristic. 



The symnietry or asymmefry of the valve bas been considered by Heiberg, Pfitzer and others 

 as of great importance in the formation of families. But this can hardly be maintained, since we 

 meet "vrith forms both symmetrical and asymmetrical in the same species, as for instance Trachy- 

 neis apera, Frusftilia vidgaris and others; and there are asymmetrical species closely allied to .sym- 

 metrical in the section Navicidce lineolatce, Finnidaria, Caloneis and others. The degree of asym- 

 metry is also variable. This tends to shew that asj^mmetrical forms are derived from symme- 

 trical and vice versa, and that in a natural system they cannot be placed in different families. 

 As a generic distinction it may, when combined witli other characteristics, be of value in eer- 

 tain cases. 



A median constricfion of the valve has been usually considered as an important character- 

 istic, but it is not so, panduriform species being closely connected with others whieh are not at all 

 constricted. Dictyoncis marginafa, for instance, is usually deeply constricted, bxit varieties exist 

 connecting this species with the non-constricted D. Thnmii, which may therefore be considered as 

 its variety. In the genus Diploneis very closely connected, constricted, and non-constricted forms 

 exist. Fiploneis didyma is i;sually panduriform, but in slightly brackish aväter the constriction 

 becomes less distinet and, finally, not perceptible. "We conclude thiis tiiat the outline of the valve 

 in this respect is too variable to furnish generic distinctions, thongh in many cases, when constant, 

 it may be a valuable specific characteristic. 



Central nodule. The central nodule constitutes a more or less conical, interiör silicious mäss 

 in the centre of the valve. In many cases it expands laterally into a stauros, and on this charac- 

 teristic has been founded the genus Stauroneis. It is to be remarked that a staiiroid nodule occurs 

 in widely different forms, and that some really allied forms scarcely dift^er otherwise than in the 

 presence or absence of a stauros. In such cases the stauros cannot be iised as a generic distinc- 

 tion. As a specific characteristic it is in most cases very valiTable, as it is very constant in the 

 same species. 



The central nodule extends not only transversely, but is in many cases prolonged into Jiorm, 

 as in Diploneis, in which they closely follow the median line. In other cases, as is Dictyoncis, 

 larger foi-ms of Stauroneis, Frustidia, the median line is enclosed betweeu silicious ribs, in the 

 middle frequently fused together with the central nodiile, which thus seems to be elongated. The 

 prolongation of the central nodule in such a manner reaches its maximum in Amphiplcura, where 

 the -iforliSi represent the horns of Diploneis. In the section NariculcB lyratce the horns are distant 

 from the median line and developed into the 13'riform hiarkings, or large lateral. lunate areas, 

 characterizing this section. The characteristics of the central nodule are subject to only slight 

 variation and are therefore of importance. 



Median line. The median line is in all probability a fissure of more or less complicated 

 structure, and seems to afford valuable characteristics. In most of the Mastogloim it is undulating, 

 also in Navicida plicatida. In Fleurosigma, Gyrosigma. and others, it is more or less sigmoid. but 

 this characteristic, thongh valuable specifically, is not of sufiicient constancy to afford generic dis- 

 tinction, sigmoid median lines occui^ring in forms nearh- akin to others with straight median lines, 



