Seldom, indeed, do we meet with crustacea of any great size in the very old rocks, unless these be members of the Trilobite order. Of these, indeed, the largest forms are the oldest of all, for no Trilobite approaches the size of the giant Paradoxides, which characterize the Menevian group—the oldest fossil group with which we are acquainted—for, in spite of all that has been said and done, Eozoon has not passed beyond the mythical stage. Some of its admirers regard it at least with superstitious reverence. But into the positive stage it has not entered; and, therefore, the Menevian group, proposed by Dr. Hicks and myself for Britain, and recognized abroad, is the oldest known. No Crustacea, except a few small Bivalve Phyllopods (Leperditia), are known to accompany the trilobites in it. But when we ascend to the next overlying group—the true Lingula flag or Pfestrinog group of Sedgwick, while the trilobites reduce their size, a large Phyllopod or two, Hymenocaris, usurps the ascendancy, and, up to yesterday, was probably the most active, and therefore the highest, of his tribe, known in these rocks. Lately, Mr. David Homfray, the explorer of the Lingula and Tremodoc groups (Sedgwick), has struck out a still larger, possibly a rarer form, of the great shield-bearing Phyllopoda. Instead of having an ample folded carapace, or shield, like Nebalia, it had a hollow oblong scute, to all appearance only slightly hollowed, after the manner of Apus. Three distinct ridges on the hinder ...