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CEUSTACEAN TRACKS IN POTSDAM SANDSTONE. 



The following- interesting letter by Professor Hall (31st Oct. 18G2), 

 " On a New Crustacean, from the Potsdam Sandstone," was pub- 

 lished in the December number of the ' Canadian Naturalist.' 



I have been much interested in reading jour 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, without 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 yon 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. Barrande. In 1855, I obtained 

 from the Potsdam Sandstone of the Upper Mississippi Eiver, 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 w T orth 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 Ilivcr, in that State, tracks similar to 

 those described by Sir W. E. Logan iu the sandstone of Canada. This 

 added a new interest to the unknown crustacean fragment; and in 18G0 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 18(52 



Fig. I. — Shield of Crustacean, from Primordial Zone. 



sent my assistant in the Wisconsin survey, M.r. Hale, to make further ex- 

 plorations, but he did not succeed in finding anything of interest. At 

 another locality, however, he obtained sonic fragments of the crustacean 

 before mentioned* among which arc two cephalic shields sufficiently perfect 

 to be characterized. 1 enclose yon a drawing of one of ihese. 



* 'Canadian Naturalist' for August, 1862. 



