PERHAPS the most enduring and abundant of the animals without backbones have been the Mollusca, or shell-fish, which include not only the common forms familiar to us all to-day but also some extraordinary forms in the past which call at least for mention here.
The most important of them were the ammonites whose serpent-like coiled shells are so common in our Dorset and Yorkshire coasts. These coil-like shells, when sectioned, show that the interiors consisted of a number of communicating chambers similar to those in the shell of a modern nautilus (or one-valved animal). The occupant, a kind of cuttlefish, lived in the last chamber, and it is very probable that the other chambers were filled with gas so that the whole served as a float.
Closely related to the ammonites are the fossils known as belemnites. The narrow cone-shaped fossils of that name are really the same sort of thing as the ‘pen ‘of the modern squid, although they are more solidly built. This cone is, of course, merely the horny and internal skeleton of a cuttlefish, and though the belemnites are common and well known, only in a few instances have we any knowledge of the soft-bodied creature itself.
Perhaps one of the most astonishing fossil molluscs, if indeed it is a shell at all, is the gigantic spiral form whose remains were discovered in Alexandra Park, Hastings, some years ago. Actually indications of two individuals were obtained, one with a right-handed, and one with a left-handed, spiral. From the clues as to size, which the fragments of one of these gave, the whole structure was completed in plaster, and a great but moderately thin spiral shell, over seven feet high, resulted. It is now exhibited in the geological department of the British Museum. This was described as a fossil under the name of Dinocochlea, or ‘huge-shell,’ but it has been the subject of much controversy, as many people maintain it to be merely
some curiously shaped mass of materials which had grown together. This diagnosis is very probably correct, although how such a concretion could be formed is a problem likely to be perpetually debated by geologists.
Along with these molluscs the somewhat similar Brachio-poda, or lamp-shells, continued to exist. Occasionally they were very numerous, but like the molluscs they have succeeded in coming down through the jeons of geological time and have living representatives, so that no special attention need be paid to them here. It may be pointed out, however, that their popular name of ‘lamp-shell ‘is derived from the similarity between the shell and a Roman lamp.
THE ARRIVAL OF THE INSECTS
THE Arthropods, the highest of the invertebrate group, to which belong the trilobites already mentioned, continued to flourish and to produce new and complex forms. The insects, for example, first came upon the scene definitely in the Carboniferous period.1 They must have been in existence before then, and are probably contemporaneous with the earliest land plants, but it is interesting to see that in the Carboniferous period, with its prolific plant life and perhaps dense atmosphere, the marine arthropods like the trilobites and the eurypterids were dying out, while on land the new arthropods of the air, unhampered by competition as there were yet no birds, were making a conquest that they have not yet relinquished. Remains have been found of dragon-flies of this age whose wings are two and a half feet in span. At this time, too, were the swamps, the luxuriant vegetation, and the generally warm temperature which, combined together, have given us our seams of coal.