84 
Edwards^ 07i Diatomacece. 
and the admirable adaptation of each to its especial purpose. 
The triradiate spicnla_, however modified by size or form^ are 
essentially skeleton spicula ; while the simple acerate form 
appertain more especially to the defences of the animal. 
In both classes of spicula great latitude in size prevails^ to 
adapt them to their situation and purposes in the structure 
of the animal. 
On DiATOMACEiE Collected in the United States. By Arthur 
M. Edwards_, Member of the New York Lyceum of 
Natural History ; Member of the Academy of Natural 
Sciences, Philadelphia, Pa. 
(Read March 30tli, 1859.) 
Availing myself of the kind oflPer of F. C. S. Eoper, Esq., 
to lay before you any facts relating to the family of the 
Diatomacese that I might forward to him, I now present a 
list of the species I have detected during my searchings for the 
last three years. 1 am in hopes that the fact of these being 
lists of species found in the United States, N. A., will excuse 
the otherwise unimportant nature of the investigations 
made. I have no ncAV facts to adduce, nor any new species to 
describe, but one or two curious forms have been found that 
will serve to illustrate the great variability of outline in some 
of the species of the Diatomacese. When bringing forward 
these facts, I lay down no rules and attempt to manu- 
facture no new laws to govern the formation of species, nor 
do I wish to mix myself up with the very complicated dis- 
cussions now going on among microscopists on this sub- 
ject. That many false species have been manufactured by 
superficial observers cannot be denied. This interesting 
and important branch of science has thus been much com- 
plicated, and botanical works crowded with worse than useless 
synonyms. No species can be firmly established from an 
examination, however careful, of frustules prepared by means 
of acid, and its natural history can only be studied from the 
living plant. In such cases as the examination of the 
Charleston Harbour-mud, mentioned below, only well estab- 
lished species are set down, the doubtful ones having been 
omitted, for the reason I have given below. 
The first gathering I shall mention is one taken from 
around the roots of Valisneria spiralis at Fishkill Creek, 
