287 
On Several New and Rare Species of Fresh-water 
Diatom ao® discovered in Northumberland. By Arthur 
Scott Donkin, M.D., Lecturer on Forensic Medicine to 
the University of Durham, and Physician to the Sunder- 
land Infirmary and Dispensary. 
With Plate XVI II. 
In pursuing my investigations with the view of publishing, 
at no distant period, an entirely new work on the British 
Diatomacese, I have recently met with several highly in- 
teresting fresh-water species; some of them entirely new to 
science, the others scarcely known to observers in this country. 
To a description of these I now beg to direct attention. 
In my previous contributions on the Diatomaceae to the 
Royal Microscopical Society of London , 1 and to the pages of 
this Journal , 2 I have fully indicated the localities where 
immense congeries of marine species may be found luxuriating 
in their natural habitat on the sandy shore, during the warm 
months of summer. I now venture to offer a few observa- 
tions on the localities most favorable to the production of 
fresh-water species, and the best method of collecting them ; 
but in our search for these we must take our leave of the 
delights and refreshing breezes of the sea-beach, where — 
“ Percussa juvant fluctu tam lit.ora,” 
and direct our steps to meandering rivers and the babbling 
waters of mountain streams — 
“ Quae 
Saxosas inter decurrunt flumina valies,” 
with all their fascinations for the student of Natural History. 
It is a mistaken assertion, and only applicable to the com- 
moner species, that the Dialomacem are almost ubiquitous. 
On the contrary, many species are extremely fastidious as to 
localities, while others are only propagated in abundance at 
certain seasons of the year. Thus the humble Gomphonema 
olivaceum only occurs in the spring, in small clear streams 
and ditches in exposed situations, covering the stones at the 
bottom with olive-coloured mammillated cushions, and only 
in those places where the water is running with some degree 
of swiftness. Remove a mass of this species from its habitat, 
and place it in a bottleful of the water in which it was for- 
1 ‘ Trans. Micr. Soc.,’ vol. vi, New Series, p. 12. 
2 Vol. i, New Series, p. 1. 
