E. M. NELSON ON THE NODULE5 IN TLEUROSIGM.E. 163 



one side exhibits a tendency to turn down, this slight displacement 

 being jiibt perceptible. Care must be exercised always to examine 

 a specimen that is lying fiat, for if it is tilted fallaceous appear- 

 ances will be present ; also it is better to examine these structures 

 from the outside, because an inside view usually presents more 

 difficulty. 



It may be said that this is only a matter of sKght variation, 

 such as is to be met with in all the diatomacese ; but twenty-four 

 varieties of Fleurosigmce have been examined, and not a single 

 instance of the ends of the pipes pointing to one another has been 

 observed. The following is a list of the twenty-four varieties of 

 Pleurosiymce : — 



P. formosum, decorum, elonyatum, striyosum, rhomheum, 

 convexum, qimdratum, angidatwni (3 varieties), cesttcarii, navicic- 

 laceum, balticum (3 varieties), strigilis, attenuatum, hippocampus 

 (2 varieties), acuminatitm, scalpi'oides, offine, eximum, affine var. 

 fossilis, Richmond, U.S.A. 



In addition to these F.fasciola and littorule were examined, but 

 the structure in both these forms is so minute that it could 

 not be traced. P. spectahile has such a deep nodule that the 

 structure on the upper surface could not be seen by an inspection 

 from the inside ; a valve mounted the other way up was not at 

 hand. 



The ends of the pipes in P. formosum are shown in fig. 8 ; they 

 are not very easy to make out, for although it is a very large 

 plewosigma it has nevertheless a rather small nodule. P. decoritm 

 is merely a replica of P. formosum. P. rigidum is a very difiicult 

 one to observe : it has one end of its raphae pipe straight, the 

 other comes straight towards it, and then tui-ns very slightly to 

 one side. 



In exterior shape the Maryland Pleurosigma and P. affine var. 

 fossilis Kichmond are very similar to the P. angulatum, met with 

 in common spread slides, but in P. affine the extremities of the 

 valve are blunter and more like P. rigidum. 



With regard to the sigmoid curve of the rapha^, that of 

 P. rigidum is the straightest, P, affine is straighter in the centre 

 and curves more rapidly at the extremities, while in P. affine var. 

 fossilis, the Maryland j^lGurosigma and P. angulatum, the curve of 

 the raphae has a more regular sweep throughout. In fact, these 

 three diatoms — viz. P. affine \2ii\ fossilis, the Maryland ^j'Zei«W^7?i« 



