206 



1. Eunotia incisa, n. sp. 



with 2 varieties. 



2. Pinnularia latestriata, n. sp. 



2 varieties. 

 8. Cymbella , n. sp, 



4. Gomphonema Brebissonii, n, sp. ? 



5. „ hebridense, n. sp. 



6. Stauroneis rectangularis, n. sp. 



7. Pinnularia exigua, n. sp. ? 



8. „ undulata, n. S2). 



9. „ parva, n. S2). ? 

 10. „ tenuis, n. sp. 



11. Tryblionella angustata ? 



3 varieties of this known species, if 

 not of a new one. 



12. Navicula spiculata, n. sp. 

 Discovered by the Rev. \V. Smith, in 



the living state at Grasmere, but 

 not yet described. The author also 

 found it in the Mull deposit. 



13. Pinnularia divergens ? 



Several very remarkable varieties 

 which the author referred, with 

 some doubt, to this species, lately 

 established by Mr Smith. 



Having thus described about 140 species in the above three 

 categories, the author stated, that some additional forms, not yet 

 precisely determined, would have to be added to each ; and he next 

 proceeded to make some general remarks on the value of generic 

 and specific characters in the Diatomaceee. 



He showed that some genera had been established on apparently 

 insufficient grounds ; thus, Eunotia is separated from Himantidium, 

 because the latter occurs in chains, the former solitary. But Eunotia 

 tetraodon is found in chains, both alive and in this deposit ; and if 

 we transfer it to Himantidium, we separate it from Eunotia Dia- 

 dema, to which it is so closely allied. The author concluded that 

 these two genera should be united. 



Again, Cocconema is separated from Cymbella by the former 

 having a stipes, the latter not. But this seems a very slight 

 foundation for a genus where the frustules cannot otherwise be dis- 

 tinguished, as in this case ; and here also the author would unite 

 the two genera. 



In regard to specific characters, the author showed that those 

 usually resorted to, such as form, size, number, and arrangement 

 of striae, &c., are subject, in certain species, to almost unlimited 

 variation, of which he gave a striking example in Eunotia triodon, 

 and others in Pinnularia divergens, Eunotia bigibba, and Himanti- 

 dium bidens. In other cases, again, the species never varies except 

 to a small degree in size. This was shown in Eunotia tetraodon 

 and E. Diadema, and mentioned as occurring in Epithemia gibba, 

 Navicula serians. Amphora ovalis, Pinnularia alpina, P. lata, and 

 many others. It therefore appears that the tendency in a species 

 to vary may be regarded as itself a specific character, as may also 

 the absence of this tendency. 



With regard to the actually admitted genera and species, the 



