6 Rev. W. Smith on the Diatomacese, with descriptions 



carefully executed di'awing of this species under a power of 5500 

 diameters ; I give a portion of it in Plate T. fig. 2 : he remarks, 

 that " the figure of the frustule, produced by the twist in the 

 valves, might be represented by joining together the tip ends of 

 two feathers taken from opposite wings. ^^ 



Plate I. fig. 1. A valve of P. formosurri', fig. 2. a portion of the same 

 magnified 5500 diameters. 



2. Pleurosigma speciosum, n. sp. Valve linear, slightly attenuated, 

 somewhat twisted, extremities obtuse, median line not central : 

 colour a pale straw. Average length of valve ^^j of an inch, 

 greatest breadth of ditto ^J-^ of an inch. Strise 4^1^00 ^^ ^^ 

 inch apart {v. s.). 



Locality unknown. 



A slide containing this species was sent to me by Messrs. 

 Smith and Beck, but without any precise information as to the 

 locality where the specimens had been found. It is a near ally 

 of the former, but of a straighter and stouter habit and more 

 delicately marked ; it wants however the gracefulness of the cur- 

 vature so eminently characteristic of the species first described. 



Plate I. fig. 3. A valve of P. speciosum. 



3. Pleurosigma elongatum, n. sp. Valve linear-lanceolate, flexure 

 slight, extremities acute, median line central : colour a clear 

 straw. Average length of valve -fj of an inch, greatest breadth 

 of ditto -gl^ of an inch. Strise ^4^00 ^^ ^^ ^^^^ ^P^^^ (^- ^O* 

 Poole Bay, 1848. Brackish water near Lewes, 1850. Hull, ilfr. 



R. Harrison ! 



Not uncommon in pools at half-tide level, and in brackish 

 ditches mixed with other species. It has been distributed by 

 Mr. Harrison under the name of '^ Navicula lineata,^^ but this 

 term, if at all admissible, approaches too nearly the " lineolata '^ 

 of Ehrenberg, which refers to a species wholly different. 



Plate I. fig. 4. Valve of P. elongatum. 



4. Pleurosigma delicatulumj n. sp. Valve narrow lanceolate, 

 flexure slight, extremities acute : colour very pale straw with 

 a slight tinge of pink. Average length of valve ~-^ of an inch, 

 greatest breadth of ditto y jq ^ of an inch. Strise joiiT^ ^^ ^^ 

 inch apart [v. v.) . 



Brackish water ; Lewes, 1850. 



Very close to the last in outline, but much more delicate, and 

 easily distinguished by its size and colour : it requires the most 

 careful manipulation to discover the strise with the power I have 

 employed. 

 Plate L fig. 5. Valve of P. delicaiulum. 



