Page:A Treatise on Geology, volume 1.djvu/92

 76 Aymestry, and in most of the limestones of the Devonian and carboniferous rocks. The magnesian limestone has a small number; certain oolites are full of them; the green sand and chalk yield great plenty of sponges; the calcareous and arenaceous tertiaries of France furnish many beautiful forms, of genera often the same as those now found in the sea. Undoubtedly, as a general rule, zoophyta occur more plentifully in calcareous rocks than in any others; they are probably more numerous in the older strata; and there are probably more fossil than recent species, if we exclude from the latter, those whose bodies are unconnected to stony or horny external or internal supports.

It was once imagined that the higher orders of zoophyta, those ranked by Lamarck in his group of echinodermata, were absent from the older formations; and certainly they are, at least, not common among the very oldest fossils. Crinoidea, however, occur in the silurian rocks, and they are more plentiful in the carboniferous limestone than in any older or more recent deposits. Echinida and Stellerida first appear in the lower silurian rocks (Malvern and Wales), but become far more numerous in the oolitic and chalk systems. Sponges are by far most numerous in the cretaceous rocks.

In the above table, the small stars indicate that some species of the groups of zoophyta whose names occur above are found in the system of strata on the line of which they are situated; the large stars are placed on the line of that system of strata in which the group of zoophyta is specially numerous.