iSgi.] NEW-YORK MICROSCOPICAL SOCIETY. 67 



escaped attention. There is first the difference of curve between 

 the two sides of the valve — flat on the inner and convex on the 

 outer — common to all, and with it the different appearance pre- 

 sented optically; secondly, all the valves of this species character- 

 ized by diagonal *' markings " have a row of coarser perforations 

 running parallel to the margin of the valve, and also a row of 

 similar ones on each side of the median line, making altogether 

 four rows on the face of each valve ; and, lastly, the nodule shows 

 a characteristic difference on the two sides of the valve. On the 

 one side it seems simply a cavity with the larger perforations 

 running around, while on the other side it seems to be formed as 

 a ring connecting the two halves of the median line. The four 

 parallel rows, however, of larger perforations present the most 

 characteristic features in common, and many of the specimens 

 thrown on the screen from J^. formosicm downward will show 

 them distinctly. 



The finer specimens will mostly explain themselves, coming 

 after P formcsutn ; for convenience of comparison with which I 

 have taken some of the latter at half the number of diameters, 

 that the detail may appear of about the same minuteness as the 

 finer forms. Except that of P. angulatum, all my negatives have 

 been taken from two slides of P. formosutn, and for a long time I 

 was unable to get any positive evidence of the finer structure of 

 P, angulatum much different from the usual appearance, although 

 there was no doubt about its resemblance in the general features 

 to the coarser forms. But latterly I have been more fortunate, 

 and am able to present a series of slides of that diatom which 

 establishes beyond a doubt the identity of its structure with the 

 other forms of the genus to which it belongs. Altogether, then, 

 we have examples of four different species, differing vastly in size 

 and shape, but in the finer structure all present the same features. 

 I admit that examples of torn structure in the last three typical 

 forms, although numerous enough to prove my point, do not offer 

 the same extent of torn surface to study from as P. formosutn ; 

 but, under certain circumstances, this would be scarcely possible. 

 P. formosum has the structure twice as coarse as the other forms, 

 and the mechanical pressure exerted on them in mounting being 

 the same in both cases, what would be sufficient force to separate 

 the fibrils only, and still leave them sound in the one, would 



