The present memoir is the result of the examination of an Ichthyolite discovered by Mr. Boscawen Ibbetson in the Isle of Wight, near the junction of the Lower Green Sand with the Wealden, and sent to me in the hopes that it might tend to show to which of the two formations this bed should be assigned. The evidence it affords on this question is neither direct nor conclusive, inasmuch as it is an undescribed species, and consequently any deductions beyond those based upon general affinities would be unwarrantable. In another point of view, however, this specimen is of high scientific value, as it sets at rest the long-mooted questions of the relative characters of the upper and lower teeth, and their general contour in the individuals composing the genus Hybodus so extensively occurring in the secondary strata. Mr. Ibbetson has had the rare fortune to bring to light the entire mouth of a fish of this genus. The left side is slightly crushed, but the other retains its natural form, and the greater portion of the teeth in both the upper and the lower jaw. The former measures 10 inches, and appears to have carried twenty-four teeth in the front series; the latter measures 7½ inches, and has nineteen teeth in series, one on the symphysis and nine on either side. Two rows of succession teeth are traceable behind the front series.