The following interesting letter by Professor Hall (31st Oct. 1862), “On a New Crustacean, from the Potsdam Sandstone,” was published 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 Potsdam Sandstone; more especially as these observations connect themselves in a remarkable manner with a recent discovery of my own; and a question 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 History 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 subsequently I sent a drawing of the same to M. Barrande. In 1855, I obtained from the Potsdam Sandstone of the Upper Mississippi River, 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.