155 
ORIGINAL COMMUNICATIONS. 
Descriptions of Diatomacrex observed in CaLirorNiaNn Guano. 
By R. K. Grevitxz, LL.D., F.R.S.E., &e. 
Ir is well known that in studying Diatomacee as they 
occur in guano and in deposits, the observer labours under 
great disadvantages. The materials he has to work upon 
are often scanty, and forms which attract his attention are 
sometimes so scarce that he is obliged to review with the 
utmost care a multitude of preparations before he can arrive 
at a satisfactory conclusion regarding them. The conditions 
so well laid down by Professor Smith, as requisite for the 
determination of species, are in such cases, to a considerable 
extent, beyond his reach. It has consequently been a ques- 
tion whether any Diatom ought to be described except trom 
recent, and, in some genera, actually lving individuals. 
Undoubtedly it would be always desirable to conform to 
such a rule when practicable ; but so many justifiable excep- 
tions present themselves, that it can never be enforced as a 
positive law. The best writers on this most interesting 
order, even Professor Smith himself, occasionally deviate 
from it. Witness his description of Himantidium (?) William- 
sonit, made from specimens insufficient to determine even 
the genus; for he had only the front view of the frustules 
before him. Professor Gregory supplied the deficiency by 
figuring the side view, and still the genus is not settled. 
Nevertheless, it was unquestionably desirable that we 
should possess good representations of so very curious a 
production. Were we rigidly to confine ourselves to 
those species only, whose history as living vegetables we 
have the means of tracing, many of the most curious and 
beautiful forms on record—even entire genera—would have 
to be expunged from our books; such, for example as 
Heliopelta, Asteromphalus, Asterolampra, &c. The truth 
is, that while many frustules occurring in deposits require 
to be described—if described at all—with extreme caution, 
numerous others, especially of disciform genera, exhivit at 
least sufficiently marked distinctive characters. All that 
can be said, I apprehend, against the description of such 
diatoms is that from the very nature of the case we are un- 
VOL. VII. fe) 
