b p. T. CLEVE, SYNOPSIS OF THE NAVICDLOID DIATOMS. 



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

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

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

 variation. I therefore conclude that though 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 Pleurosigma and Scolio- 

 pleura. This characteristic is subject to great variation in Pleurosigma, some forms of which are 

 almost straight. On the other hand widely 'different forms of other genera are sigmoid, as Navicula 

 Baeana, Navicula Sigma, Caloneis staurophora. 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 symmetry or asymmetry of the valve has been considered by Heibbrg, Pfitzer and others 

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

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

 neis apera, Frustulia vulgaris and others; and there are asymmetrical species closely allied to sym- 

 metrical in the section Naviculce lineolatce, Pinnularia, Caloneis and others. The degree of asym- 

 metry is also variable. This tends to shew that asymmetrical 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 with other characteristics, be of value in cer- 

 tain cases. 



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

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

 constricted. Dictyoneis marginata, for instance, is usually deeply constricted, but varieties exist 

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

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

 exist. Diploneis didyma is usually panduriform, but in slightly brackish water the constriction 

 becomes less distinct and, finally, not perceptible. "We conclude thus that the outline of the valve 

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

 it may be a valuable specific characteristic. 



Central nodule. The central nodule constitutes a more or less conical, interior silicious mass 

 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 stauroid nodule occurs 

 in widely different forms, and that some really allied forms scarcely differ otherwise than in the 

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

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

 same species. 



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

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

 larger forms of Stauroneis, Frustulia, the median line is enclosed between silicious ribs, in the 

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

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

 the »forJcs» represent the horns of Diploneis. In the section Naviculce lyrates the horns are distant 

 from the median line and developed into the lyriform markings, 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 Mastogloiee it is undulating, 

 also in Navicula plicatula. In Pleurosigma, Gyrosigma, and others, it is more or less sigmoid, but 

 this characteristic, though valuable specifically, is not of sufficient constancy to afford generic dis- 

 tinction, sigmoid median lines occurring in forms nearly akin to others with straight median lines, 



