The great province of the Mollusca, in which, for our present purpose, we may include some aberrant and rudimentary Molluscoids, is now best known to us by its medium types, the univalve and bivalve Shell-fishes; the higher group of the Cuttle-fishes and Nautili, though not uncommon, being much less numerous, and one at least of the lower groups, the Lamp-shells or Brachiopods, being represented in the modern world by but few forms. The extension of the Mollusks backwards into the Cambrian is remarkable as being on the whole meagre in comparison with that of the Crustaceans, and as presenting only in small numbers the types most common in later times. One or two shells, and perhaps some tracks, represent the highest group: some forms resembling the floating species of Sea-snails, and a very few ordinary bivalves represent the types best known in the modern seas; while the Brachiopods, and probably some still simpler forms, are in great comparative excess. The individual specimens are also of small size, as if these creatures were but insinuating themselves on the arena of life in insignificant and humble forms. So far as yet known, the lowest groups supposed to be allied to the Mollusks, the Ascidians or Sea-squirts, and the Sea-mosses (Polyzoa), do not appear; but they may have been represented by species which possessed no hard parts capable of preservation.
This leads us to the consideration that while all the Crustacea necessarily possess some kind of crust or external skeleton, the Mollusks are very different in this respect. While some of them have ponderous shells, others even of the highest forms are quite destitute of such protective parts. This again leads to a curious question respecting the armature of the Trilobites. Some of these, even of the larger species, have strong and formidable spines, like those of the King-crabs and other modern Crustaceans. Now in the modern species we know these organs to be intended to defend their possessors against the attacks of fishes more swift and powerful than themselves. But what enemies of this kind had the Trilobites to dread? Yet species a foot or more in length presented great bayonet-like spines.
Fig. 4.—Ctenichnites ingens, Matthew.
A slab with markings of aquatic animals. From specimen in Peter Redpath Museum.
All that we know on this subject is that on the surfaces of the Lower Cambrian rocks there are in some places complicated and mysterious tracks or scratches, which seem to have been produced when the rock was in the state of soft mud, by large and swiftly swimming animals possessing some sort of arms or similar appendages ([Fig. 4]). Matthew has ingeniously suggested that they may have been large Mollusks allied to the modern gigantic Squids which still abound in the ocean, that they may have been sufficiently powerful to prey on the Trilobites, and, being swift swimmers, would have found them a helpless prey but for their defensive spines. Yet such large Mollusks might have perished without leaving any remains recognisable in the rocks, except what may be termed their hand-writing on clay. A few small examples of the shell-bearing species of these highest Mollusks, however, appear in the Cambrian, and in the succeeding ages they become very abundant and attain to large dimensions, again dwindling toward modern times. It would thus seem that for some unknown reason the highest and lowest Mollusks may have been locally plentiful, but the intermediate types were rare.
The much lower group of Echinoderms, or Sea-urchins and Sea-stars, curiously enough puts in but a small appearance in the Early Cambrian, being represented, as far as yet known, by only one embryonic group, the Cystideans. A little later, however, Feather-stars became greatly abundant, and a little later still the true Star-fishes and Urchins. The aberrant group of the Sea-slugs seems, so far as known, to be of more modern origin; but most of these animals are soft-bodied, and little likely to have been preserved.
The great group of the coral animals, so marked a feature of later ages, is scarcely known in the oldest Cambrian, except by some highly generalized forms[5] ([Fig. 5]). There are, however, small Zoophytes referable to the lower type of Hydroids, and markings which are supposed to be casts of stranded Jelly-fishes. If, with some naturalists, we regard the Sponges as very humble members of the coral group (Cœlenterata), then we have a right to add them to its representatives in the lowest Cambrian; but perhaps they had better be ranked with the next and lowest group of all—the Protozoa.
[5] Dr. G. J. Hinde has carefully studied these forms, and also similar species occurring in Lower Cambrian beds in different parts of North America, Spain, Sardinia, and elsewhere. See note in the Appendix, and Journal Geol. Society of London, vol. xlv. p. 125.
Fig. 5.—Archæocyathus profundus, Billings.
Possibly a Coral of generalized type from the Lower Cambrian of L'Anse à Loup, Labrador. A small specimen.