external features, and have been mistaken for them, but which are nevertheless Acalephs by their internal structure; for, instead of having the vertical partitions dividing the body into chambers, so characteristic of the Polyps, they are divided by tubes corresponding to the radiating tubes of the Acalephs proper, these tubes being themselves divided at regular distances by horizontal floors, so that they never run uninterruptedly from top to bottom of the body.  I subjoin a woodcut of a Silurian Coral, which does not, however, show the peculiar internal structure, but gives some idea of the general appearance of the old Hydroid Corals.  We have but one Acalephian Coral now living, the Millepore; and it was by comparing that with these ancient ones that I first detected their relation to the Acalephs.  For the true Acalephs or Jelly-Fishes we shall look in vain; but the presence of the Acalephian Corals establishes the existence of the type, and we cannot expect to find those kinds preserved which are wholly destitute of hard parts.  I do not attempt any description of the Polyps proper, because the early Corals of that class are comparatively few, and do not present features sufficiently characteristic to attract the notice of the casual observer.

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Of the Echinoderms, the class of Radiates represented now by our Star-Fishes and Sea-Urchins, we may gather any quantity, though the old fashioned forms are very different from the living ones.  I have dwelt at such length in a former article[A] on the wonderful beauty and variety of the Crinoids, or “Stone Lilies,” as they have been called, from their resemblance to flowers, that I will only briefly allude to them here.  The subjoined wood-cut represents one with a closed cup; but the number of their different patterns is hardly to be counted, and I would invite any one who questions the abundant expression of life in those days to look at some slabs of ancient limestone in the Zooelogical Museum at Cambridge, where the stems of the Crinoids are tangled together as thickly as sea-weed on the shore.  Indeed, some of our rock-deposits consist chiefly of the fragments of their remains.

[Footnote A:  See Methods of Study in Natural History, Atlantic Monthly, No.  LVII., July, 1862.]

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The Mollusks were also represented then, as now, by their three classes,—­Acephala, Gasteropoda, and Cephalopoda.  The Acephala or Bivalves we shall find in great numbers, but of a very different pattern from the Oysters, Clams, and Mussels of recent times.  The annexed wood-cut represents one of these Brachiopods, which form a very characteristic type of the Silurian deposits.  The square cut of the upper edge, where the two valves meet along the back and are united by a hinge, is altogether old-fashioned, and unknown among our modern Bivalves.  The wood-cut does not show the inequality of the two valves, also a very characteristic feature of this