MR. JAMES HORNELL, who is still in Ceylon carrying on the investigation of the pearl-oyster fisheries which I started in 1902, tells me in a letter just received that he has now succeeded in finding the final stage of the cestode larva which we found to be the nucleus of the best Ceylon pearls. We found this larva (a Tetrarhynchus), in the spring of 1902, in the pearl-oyster, and, later on, what we took to be its later stages in the file-fishes (Balistes) which feed upon the pearl-oysters, and we felt pretty certain (as I stated in the first part of my report, now published) that the adult would be found in Trygon or some other large Elasmobranch. Mr. Hornell writes from Trincomalie, November 16, as follows:—“Just a line to tell you that I have found the final host of Tetrarhynchus unionifactor.1