192 THE AMERICAN MONTHLY [June 



lying flat, for if it is tilted fallaceous appearances will be 

 present; it is also far better to examine these structures 

 from the outside, because an inside view usually presents 

 more difiiculty. 



It may be said that this is only a matter of flight va 

 riation, such as is tp be met with in all the diatomaceeB ; 

 but twenty-four varieties of Pleurosigmse have been ex- 

 aminedj and not a single instance of the ends of the pipes 

 pointing to one another has been observed. The follow- 

 ing is a list of the twenty-four varietiesof Pleurosigmse: — 



P. formosum, decorum, elongatum, strigosum, rhom- 

 beum, convexum, quadratum, angulatum (3 varieties), 

 sBstuarii, naviculaceum, balticum (3 varieties), strigilis, 

 attenuatum, hippocampus (2 varieties), acuminatum, scal- 

 proides, affine, eximum, affine var. fossilis (Richmond). 



In addition to these P. fasciola and littorale were ex- 

 amined, but the structure in both these forms is so min- 

 ute that it could not be traced, P. spectabile 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 pleurosigma it has nevertheless a rather 

 small nodule. P. decorum is merely a replica of P. for- 

 mosum. P. rigidum is a very difficult one to observe : it 

 has one end of its raphae pipe straight, the other comes 

 straight towards it, and then turns very slightly to 

 one side. 



In exterior shape the Maryland Pleurosigma and P. 

 affine var. fossilis Richmond 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 raphsB, that of 

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



