26 BRITISH DIATOMACE^. 



given by Mr. Shadbolt in the Transactions of the Microscopical Society 

 of London, vol. iii. p. 49. 



1. Arachnoidiscus Ehrenbergii, i?az7. Pseudo-nodule surrounded 

 with a double row of puncta, the interior linear, the exterior cir- 

 cular. Diam. -0051" to -0161". v.s. 



A. Ehrenhergii, Bail, sic cl. De Brebisson in lit. cum icone photographica 

 pulcherrima ! A. Japonicus, Prit. Aiiim. xxiv. 18-21. 



Marine. Ilfracombe, Mr. Ralfs. 



Supp. Plate XXXI. 256. 



Genus 10. TRICERATIUM, Ehr. 



Frustules free or attached, triangular ; valves cellular, angles elevated 

 or produced. 



The triangular form of the frustules separates this genus from all 

 its alhes, from which it is also distinguished by the absence of a 

 pseudo-nodule, the appearance of such in T. striolatum ? being pro- 

 bably accidental in the single individual from which the figure was 

 drawn. The frustules are probably at first attached to larger Algae, 

 but I have been unable to determine this point from the isolated spe- 

 cimens which have fallen under my notice. The species hitherto 

 found in Britain have occurred on the southern and eastern coasts, in 

 the mud of tidal rivers. 



1 . Triceratiiim Favus, Ehr. Cells hexagonal, angles produced into 

 horn-like obtuse processes, v.s. 



Kutz. Bacill. xviii. 11. Quek. H. C. xii. 22 «. Bail. Sound, fig. 54. Prit. 

 Anim. xiv. 43, 44. 



Marine or brackish water. Thames near Faversham, Mr. Shadbolt. River 

 Orwell near Ipswich, Mr. Hodgson. (Near Charlestown, communicated by 

 Prof. Williamson.) 



Plate V. 44. Supp. Plate XXX. 44. 



2. Triceratium alternans. Bail. Cells circular, angles slightly 

 elevated, v.s. 



Bail. Sound, figs. 55, 56, 



