PLEUROSIGMA. 



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GENUS 22. PLEUROSIGMA W. Sm, 1853. 



Frustules naviculoid, elongated, with valves 

 convex, more or less sigmoid. Raphe more or 

 less sigmoid. Striae decussate or rectangular, 

 reaching almost to the raphe. Frustules with 

 connecting zone straight, generally free, rarely 

 enclosed in mucous tubes. 



The endochrome oi-Pleurosigma presents a peculiar 

 arrangement, to which we have already referred in the 

 introduction (,L 



The genus Pleurosigma includes numerous forms 

 which inhabit all parts of the world. Prof. Cleve 

 divides it into two genera, reserving the name Pleuro 

 sigma for forms with decussating striae, and giving 

 that ot Gyrosigma to those with rectangular striae. 



The name Gyrosigma is the older of the two, 

 having been given by Dr. Hassall. The special study 

 which Professor Wm. Smith made of the group has 

 caused this name of Pleurosigma to be preserved in 

 spite of the incontestibly prior right of Hassall. 



An excellent monograph of the genus Pleurosigma, 

 by Mr. H. Peragallo has recently been published in 

 " Le Diatomiste." This work includes the genera 

 Pleurosigma, Toxonidea, Donkinia, and Rhoicosigma, 

 and forms a quarto volume of 35 pages, including 10 plates, containing 271 

 figures. This monograph is indispensable to anyone who wishes to make an 

 exhaustive study of this difficult group, or to determine exotic species. 



Fig. 50 



Pleurosigma Baltic am. 



See page 4. 



