Every such work has its defects. The faults of the present volume--faults all too obvious, I am afraid--would have been probably fewer had the writer enjoyed greater leisure. Some of them, however, seem scarce separable from the nature of the subject: there are others for which, from their opposite character, I shall have to apologize in turn to opposite classes of readers. My facts would, in most instances, have lain closer had I written for geologists exclusively, and there would have been less reference to familiar phenomena. And had I written for only general readers, my descriptions of hitherto undescribed organisms, and the deposits of little-known localities, would have occupied fewer pages, and would have been thrown off with, perhaps, less regard to minute detail than to pictorial effect. May I crave, while addressing myself, now to the one class, and now to the other, the alternate forbearance of each? Such is the state of progression in geological science, that the geologist who stands still for but a very little, must be content to find himself left behind. Nay, so rapid is the progress, that scarce a geological work passes through the press in which some of the statements of the earlier pages have not to be modified, restricted, or extended in the concluding ones. The present volume shares, in this respect, in what seems the common lot. In describing the _Coccosteus_, the reader will find it stated that the creature, unlike its contemporary the _Pterichthys_, was unfurnished with arms. Ere arriving at such a conclusion, I had carefully examined at least a hundred different _Coccostei_; but the positive evidence of one specimen outweighs the negative evidence of a hundred; and I have just learned from a friend in the north, (Mr. Patrick Duff, of Elgin,) that a Coccosteus lately found at Lethen-bar, and now in the possession of Lady Gordon Gumming, of Altyre, is furnished with what seem uncouth, paddle-shaped arms, that project from the head.[A] All that I have given of the creature, however, will be found true to the actual type; and that parts should have been omitted will surprise no one who remembers that many hundred belemnites had been figured and described ere a specimen turned up in which the horny prolongation, with its enclosed ink-bag, was found attached to the calcareous spindle; and that even yet, after many thousand trilobites have been carefully examined, it remains a question with the oryctologist, whether this crustacean of the earliest periods was furnished with legs, or creeped on an abdominal foot, like the snail. [Footnote A: As these paddle-shaped arms have not been introduced by Agassiz into his restoration of the _Coccosteus_, their existence, at least as arms, must still be regarded as problematical. There can be no doubt, however, that they existed as plates of very peculiar form, and greatly resembling paddles, and that they served in the economy of the animal some still unaccounted for purpose.]