BRITISH DIATOMACE.E. 63 



brane, and also supplies a ready means of discrimination, I have en- 

 deavoured to record it with as much precision as possible. From 

 notices forwarded by Mr. Beck, I am disposed to conclude that a 

 yellow tint in the valve indicates the presence of a distinct hexagonal 

 structure in its cellular condition, and a purple colour an absence of 

 such character. 



In several species the striae are so closely arranged — or, in other 

 words, the cellular structure is so minute, — that the power I have ordi- 

 narily employed has proved insufficient for their resolution. In such 

 cases I have availed myself of Mr. Beck's aid, who has succeeded, 

 with an object-glass of ^-inch focal length and angular aperture of 

 120°, in resolving the lines on most of the species alluded to. 



Sect. I. Striae oblique. 



1. Pleurosigma formosum, W. Sm. V. linear-lanceolate, twisted, 

 flexure considerable, extremities obtuse ; median line not central ; 

 colour hght chestnut-brown; striae 36 in -001". Length '014 i" to 

 •0178". v.v. 



W. Sm. Ann. 2nd Ser. vol. ix. p. 5. pi. i. 1. Prit. Anim. xix. 1. 



Marine. Shoreham Harbour, Aug. 1850, W. Sm. Walton, Essex, Mr. 

 C. Topping. 



Plate XX. 195. Frustule with endochrome. Frontispiece, fig. CXCV. 



2. Pleurosigma decorum, n. sp. V. lanceolate, acute, angular, 

 twisted, flexure considerable ; median line not central ; colour pale 

 chestnut; striae 36 in -001". Length -0098" to -0125". v.s. 



Brackish water. Ipswich, Mr. T. Hodgson, Aug. 1851. Felixstow, Essex, 

 Mr. C. Topping. 



Plate XXI. 196. 



3. Pleurosigma speciosum, fV. Sm. V. linear-lanceolate, obtuse; 

 flexure slight, twisted ; median line subcentral ; colour a pale 

 straw; striee 44 in -001". Length -0083" to -0152". v.s. 



W, Sm. Ann. 2nd Ser. vol. ix. p. 6. pi. i. 3. Prit. Anim. xix. 3. 

 Marine. Walton, Essex, Aug. 1851, Mr. C. Topping. 

 Plate XX. 197. 



