IN a joint paper by Mr. J. T. Cunningham and myself (Phil. Trans. vol. clxxxiv., 1893, B, pp. 765–812), we have ventured to question the accuracy of the statement made in many text-books of physiological chemistry, that guanine occurs in combination with calcium in the skin of fishes. We found that the guanine occurs in the free state. In the last number of Hoppe-Seyler's Zeitschrift fur Physiologische Chemie there is a paper by Herr Albrecht Berthe, dealing with this subject, in which he shows that the calcium so frequently found with the guanine is due to the presence of impurities derived from the tissues and the scales. Its amount depends upon that of the impurities present, and is very variable. Instead of finding 11˙76 per cent. required by the formula of “Guaninkalk” Berthe finds less than one-third of that percentage present, and even this also varies within wide limits. In the paper referred to above, we found one source of the calcium was due to the presence of comparatively large crystals of calcium phosphate, which are figured on p. 788; but there is no doubt that the bulk of it is derived from the scales.