247 
CEUSTACEAN TEACKS IN POTSDAM SANDSTONE. 
The follo;ving interesting letter by Professor Hall (31st Oct. 1862), 
" On a Neu- Crustacean, from the Potsdam Sandstone," was pub- 
lished in the December number of the ' Canadian Naturalist.' 
I have been much interested in reading your observations upon the 
tracks of Limulus in sand, and comparisons with the tracks in the Pots- 
dam Sandstone ;* more especially as these observations connect themselves 
in a remarkable manner with a recent discovery of my own ; and a ques- 
tion may arise as to whether you have described an animal which I have 
found, or I have found the animal corresponding to your description. I 
will leave you and the scientific world to judge of the facts. However, 
after what you have written, I cannot now publish what I communicated 
to the Albany Institute last winter, witliout referring to your paper ; and 
in the meantime you may lay this note before the Montreal Natural His- 
tory Society, and publish it, or such parts of it, as you please. 
In February last, I communicated to the Albany Institute a notice of a 
new crustacean from the Potsdam Sandstone of Wisconsin, and subse- 
quently I sent a drawing of the same to M. Barraude. In 1855, I obtained 
from the Potsdam Sandstone of the Upper Mississippi Kiver, a fragment 
of what appeared to be a spine of a crustacean, of very remarkable and 
peculiar structure, reminding one of that bone ; and which might at one 
time, before we had accustomed ourselves to limit the geological range of 
fishes, have been taken for an ichthyic remain. 
This fragment remained in my collection a subject of much interest, for 
I was aware, from its structure, that it could belong to no genus of Trilo- 
bites, but at the same time I did not think it worth while to publish any 
notice of it from its incompleteness. 
In 1857, Mr. Daniels, of the Geological Survey of Wisconsin, discovered 
in the Potsdam Sandstone of Black jRiver, in that State, tracks similar to 
those described by Sir W. E. Logan in the sandstone of Canada. This 
added a new interest to the unknown crustacean fragment ; and in 1860 I 
visited the Black River region, to procure, if possible, some of these im- 
pressions. I failed however in finding the precise locality ; and in 1862 
Fig. 1. — Shield of Crustacean, from Priiuurdial Zone. 
sent my assistant in the Wisconsin survey, Mr. Hale, to make further ex- 
plorations, but he did not succeed in finding anything of interest. At 
another locality, however, he obtained some fragments of the crustacean 
before mentioned, among which are two cephalic shields sufficiently perfect 
to be characterized. I enclose you a drawing of one of these. 
* 'Canadian Naturalist' for August, 1862. 
