On the genus Ocythoë ; being an extract of a letter from Thomas Say, Esq. of Philadelphia, to William Elford Leach, M. D. F. R. S

After describing a genus of Ocythoë, which the author regards as new, he observes that the Ocythoë offers a deviation from the ordinary laws which apply to the testaceous Mollusca, inasmuch as it resides only in the last volution of the shell; and as the shell does not fit the body, it appears probable that it was not made for it, more especially as there is no attachment between the shell and any part of the body. The shells that approach nearest to Argonauta are of that order: this supposition, however, is not corroborated by the habits of the animal; for all hitherto discovered of that order swim to the surface; and having no other organs of locomotion than fins, cannot glide upon the bottom. We must, therefore, suppose this to have been the habit of the animal; and yet it is hardly admissible that in that case it should have eluded the observation of voyagers, when the shell has often been found occupied by the parasite.