of British species of the genus Pleurosigma. 1 1 



Hippocampus by English microscopists. The figure given by 

 Pritchard from Ehrenberg, and apparently copied by Hassall in 

 his ^Brit. Freshwater Algae/ pi. 103. fig. 11, approaches the 

 form of the present species, but the localities assigned by Ehren- 

 berg are inland. Kiitzing, as cited above, has the same figure, 

 and quotes Ehrenberg as his authority for it, but gives a habitat 

 exclusively marine, and having submitted specimens of the follow- 

 ing species to his notice, I have his sanction for regarding it as 

 distinct. Under these circumstances I am disposed to believe 

 that the two species have been frequently confounded, and their 

 close resemblance might easily occasion the mistake; their 

 habitats are however quite distinct ; the Hippocampus is a shorter 

 and stouter species, and when full-grown specimens of each are 

 compared, presents more delicate striae, requiring higher powers 

 of the microscope, and more careful adjustment for their detection 

 and resolution into beads. 



Plate II. fig. 9. Valve of P. Hippocampus; fig. 10. ditto in a young state. 



b. Freshwater Species. 



16. Pleurosigma attenuatum, mihi. Valve elongated, lanceolate, 

 obtuse ; flexure moderate ; strise very distinct : colour purplish 

 brown. Average length of valve y^^ of an inch; greatest 

 breadth of ditto yoVo* Longitudinal strise gjuoo ^^ ^^ ^^^^ 

 apart, transverse ditto ^^^qo ^^ ^^ ^^^^ apart {v. v.). 



Navicula Hippocampus, Ehr. ? Quekett on the Microscope, p. 238. 

 pi. 8. figs. 1, 2, 3. Gyrosigma Hippocampa, Hassall's B. F. W. 

 Alg. p. 435. ISlav. attenuata, Kiitz. Bacill. p. 102. tab. 4. fig. 28 ; 

 Phy. Ger. p. 95 ; Sp. Alg. p. 86. 



Very common in streams and ditches mixed with Oscillatoriece. 

 Froome near Dorchester, 1849. Sussex in many places, 1850. Near 

 Guildford, /. B. C apron, Esq. ! Fossil in deposit from Lough Mourne, 

 Co. Antrim, described in * iVnn. Nat. Hist.' for Feb. 1850. In deposit 

 at Peterhead, Aberdeenshire, described by Dr. Dickie in ' Annals,' 

 Aug. 1848! 



In the description and figure of this species given by Mr. 

 Quekett, in other respects excellent, an error has been committed 

 in representing the beads as transversely confluent towards the 

 margin of the valve. Mr. Kingsley has judiciously remarked to 

 me, that '^ this mistake is occasioned by the great convexity of 

 the shell, which causes an apparent overlapping of the beads at 

 the edge ; they are however perfectly distinct and may be defined, 

 though not easily so.^' 



Plate II. fig. 11 . Valve of P. attenuatum; fig. 12. ditto of a young speci- 

 men; fig. 13. P. attenuatum in front view showing self-division; fig. 18. 

 portion of fig. 11 magnified 3200 diameters (Rev. W. Kingsley). 



