It is not difficult to imagine the conditions under which such impressions may have been preserved, for at the present day there are to be seen, on some shores, illustrations of similar operations. Dr. Gould, of Boston, U.S., was the first to call the attention of naturalists to a very instructive example of such processes on the shores of the Bay of Fundy, where the tide is said to rise in some places seventy feet high. Here we have a very perfect surface for receiving and retaining impressions. Vast are the numbers of wading and sea-birds that course to and fro over the extensive tract of plastic red surface left dry by the far retreat of the tide in the Bay of Fundy. During the period that elapses between one spring tide and the next, the highest part of the tidal deposit is exposed long enough to receive and retain many impressions; even during the hours of hot sunshine, to which, in the summer months, this so-trodden tract is left exposed, the layer last deposited becomes baked hard and dry, and before the returning tidal wave has power to break up the preceding one, the impressions left on that stratum have received a deposit. A cast is thus taken of the mould previously made, and each succeeding tide brings another layer of deposit. We can easily imagine that in succeeding ages the petrifying influences will consolidate the sandy layers into a fossil rock. Such a rock would split in such a way, along its natural layers of formation, as to show the old moulds on one surface, and the casts on the other.
[Illustration: Fig. 16.--Limb-bones of _Allosaurus_. (After Marsh.)
Professor Marsh has had the good fortune to discover a very peculiar new form of carnivorous Dinosaur, to which he has given the name Ceratosaurus,[14] because its skull supported a horn. But the horn is not the only new feature presented by this interesting creature. Its vertebræ are of a strange and unexpected type; and in the pelvis all the bones are fused together, as in modern birds. Externally, also, the Ceratosaurus differed from other members of the carnivorous group, for its body was partly protected by long plates in the skin, such as crocodiles have: these extended from the back of the head, along the neck, and over the back. An almost complete skeleton was found which indicates an animal about seventeen feet long. When alive it was probably about half the bulk of the Allosaurus mentioned above. (See Fig. 16.)