1899] MICROSCOPICAL JOURNAL. i95 



the aperture, as heretofore alluded to. The smallest of 

 the six forms of N. firma examined is named var. 

 hitchcockii, and it has the most diverging pipe ends of 

 all : so much is this the case that, instead of finding their 

 way into the central nodule, they pass outside it into a 

 primary areolation on either side of the nodule (fig. 9). 

 Now this diatom, as well as the N. tumescens, are fossils, 

 so it would appear that these Naviculacese had put on this 

 peculiar adaptation, while the Nottingham Pleurosigmae 

 were only thinking about it. 



These observed facts naturally give rise to the follow- 

 ing questions. It is obvious that the Maryland Pleuro- 

 sigma is a very old form, so also is the P. affine var. 

 fossilis : as these have straight or nearly straight raphse 

 pipes, and also nodules of the first group, may not these 

 structures be taken as indications of early types ? If this 

 is the case, may we not conclude that the varieties of 

 Pleurosigmae named in the first group are survivals of 

 this old type, and may not those mentioned in the second 

 group be later forms ? Thus, for example, may we not 

 consider P. rigidum as the most perfect survival of the 

 oldest type of Pleurosigma yet known, because it has the 

 same form of nodule and straighter raphae pipes than 

 any of the more recent forms ? 



This is one of the most beautiful and interesting, as 

 well as instructive, of microscopic structures. The prin- 

 cipal view of the valve shows an elongated isosceles tri- 

 angle, having three bands running its whole length. The 

 outer bands are sieve-like structures; the minute holes 

 being closer together in the transverse than in the longi- 

 tudinal direction. 



In common parlance it would be said that the longitu- 

 dinal striae were finer than the transverse ; the transverse 

 striae vary, however, being finer at the wide end of the 

 valve, where they count 53,300 per inch, and coarser at 

 the small end. This agrees with the law of diatom for- 



