s, 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.

[Illustration]

[Illustration]

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.]

[Illustration]

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