MR. HARDMAN'S observations on the occurrence of “sea-fish in fresh-water rivers” (NATURE, vol. xxix. pp. 452–53) are not by any means unique, as he has supposed. On the contrary, cases similar to those he has recorded are so frequent as to justify him in believing that “some caution must be observed in the classification of strata as fresh-water or marine on the evidence of fish alone.” The incursion and confinement of the two types specially mentioned—the “sunfish” and “shark”—in fresh water have many parallels. For instance, in NATURE, vol. xiii. p. 107, Messrs. W. W. Wood and A. B. Meyer have recorded that “near Manila is the Lacuna de Baij, a large sheet of water” whose “water is quite fresh, and, after settling, perfectly potable,” but in which live a sunfish (Pristis perotellii) and a small shark. Further, in Lake Nicaragua, whose mean height above mean tide in the Pacific and Atlantic Oceans is 107.63 feet, are likewise found a sunfish—apparently Pristis antiquorum—and a peculiar shark—Eulamia (or Carcharias) nicaraguensis. The last have been especially noticed in a “Synopsis of the Fishes of Lake Nicaragua, by Theodore Gill, M.D., and J. F. Bransford, M.D., U.S.N.,” in 1877 (Proc. Acad. Nat. Sci. Phila., pp. 175–91). Therein it is also urged that “these instances, supplemented as they are by many others, are sufficient to convey a caution against too extensive generalisation of the physiographical conditions hinted at by fossil remains of aquatic types.”