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



carefully executed drawing 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 wing3." 



Plate I. fig. 1. A valve of P. formosum; 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 ^ of an inch, 

 greatest breadth of ditto ^^ of an inch. Stria? ^ <joq °f an 

 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 y'j of an inch, greatest breadth 

 of ditto -g^Q of an inch. Striae -$^xj~Uo °^ an mcn a P ar ^ i v - v -)- 

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



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 delicatulum, n. sp. Valve narrow lanceolate, 

 flexure slight, extremities acute : colour very pale straw with 

 a slight tinge of pink. Average length of valve T y ^ of an inch, 

 greatest breadth of ditto T3 L _ f an inch. Stria? jy^o q °f an 

 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 stria? with the power I have 

 employed. 

 Plate I. fig. 5. Valve of P. delicatulum. 



