454 Mr. J. Ralfs on the Diatomaceee. 



longer specimens, when the striae are obscured by the endochrome, 

 somewhat resemble D. tenue ; but in this species the frustules are 

 generally broader in proportion to their length, and the lateral 

 striae which give a punctated appearance to the margins of D. tenue 

 are wanting. 



The shorter frustules are very similar to D. flocculosum-, there 

 are however not more than two striae on each side, and the canal, 

 which is much less inflated, is but little broader than the incras- 

 sated ends. 



This species, like D. tenue, is often found with the frustules 

 thrown quite back. In this state it is figured by Lyngbye and 

 by Agardh placed in the same section as D. tenue. It is brown 

 when recent, and becomes paler when dried. 



6. D. marinum, Lyngb. Frustules varying from nearly square to six 

 times as long as broad ; canal not inflated ; striae two. D. mari- 

 num, Kutz. in Linnsea 1833, p. 584; Ag. Consp. Diatom, p. 53; 

 Hook. Br. Fl. vol. ii. p. 405 ; Harv. Br. Alg. p. 201 . D. teniaforme, 

 Ag. Consp. p. 54. D. brachygonum, Hook. Br. Fl. p. 406. Con- 

 ferva teniceformis, Eng. Bot. t. 1883 ! Tessella interrupta, Ehrenb. 

 Infusoria, p. 202 ? Pritch. Infusoria, p. 220 ? 

 Plate IX. fig. 5. a, front view ; b, side view. 



Common on marine algae. Dark brown when recent, greenish 

 when dry, gradually becoming paler. 



This species is very variable in the length and breadth of its 

 frustules ; they are generally two or three times longer than broad ; 

 but most specimens contain some frustules nearly square, and 

 others from four to six times longer than broad. 



The lateral view shows the frustules nearly plane, sometimes 

 slightly convex, and the canal not inflated. The colouring matter 

 is darker, and the granules are larger than in the other British 

 species, so that the striae can scarcely be seen when the plant is 

 recent. 



The mucous substance which forms the connecting medium be- 

 tween the angles of the frustules is more developed in this than 

 in the other species ; it is consequently more difficult to obtain 

 them in a state of separation so as to observe them laterally. 



The best method of separating them is to submit a portion to 

 a red heat, which destroys both the connecting hinge and also the 

 internal colouring matter, and thus has the further advantage of 

 rendering the striae more perceptible. 



On a front view the longer frustules resemble those of D.fene- 

 stratum ; but D. marinum, besides its much deeper colour, is more 

 rigid; its frustules are broader; its striae also are nearer the margin, 

 and most frequently have a bend towards it near the end of the 

 frustule. 



I am indebted to Mr. Borrer for a specimen oiConf. teniceformis, 



