The prevalence of Zoophyta is connected with the great abundance of limestone in this Zone, a circumstance less favourable to Crustaceans, though these are still numerous; all Entomostracous, mostly Trilobites. The Monomyarians are all Aviculoid, the Dimyarians mostly Arcaceæ and Mytilaceæ. In the Ludlow Zone which follows, we have to remark a general analogy to the Caradoc deposits, both being for the most part arenaceous and muddy deposits, with only partial expansions of the calcareous element. Hence fewer Zoophyta and Echinodermata, more Monomyaria and Dimyaria, these bivalves being still of the same natural families. In the uppermost layer Land-plants and Fishes appear for the first time in the History of Life. Plants probably terrestrial 1 genus ......1 species. marine 2 genera......5 ... دو Amorphozoa (none) The decline of the whole series is very marked in the Passage-beds, which yield only the following meagre list. Plants probably terrestrial-Lycopodiaceæ? Crustacea 5 genera........ ... 11 species. Of the few species here noted, three Fishes, the two Mollusks, and six of the Crustacea, are also found in the upper Ludlow beds, of which indeed these are merely the capping. This decline of the Silurian species may be compared with the dawn of the series in the Lingula Zone; the analogy of conditions is maintained by the Crustacea and Brachiopoda, but difference of time is marked by Gasteropoda and Fishes. From the data thus collected we may compile one general table representing the numerical prevalence in time of each of the classes of Marine Invertebrata in the Lower Palæozoic Strata as at present By inspection of this table it appears that the earliest system of marine life contained a few examples of five great classes; viz. Zoophyta, Annelida, Crustacea, Polyzoa, Brachiopoda. In the next period all the ordinary classes of Mollusca are added, in small numbers. In the third period, Echinodermata appear, and perhaps Amorphozoa, for this seems, to me at least, still somewhat doubtful. Thus excepting Cirripedia which up to this time | Heteropoda. Pteropoda. Cephalopoda. have not been certainly recognized, all the important classes of Marine Invertebrata are traced into the Lower Palæozoic Strata, beginning in each case with few species and very few genera. The progress of the several classes is very unequal. Crustacea, relatively abundant in every stage, reach a maximum in the third period. Brachiopoda, also a very abundant group, reach the maximum in the fifth period with Zoophyta and Echinodermata, while the Monomyaria, Dimyaria, Gasteropoda, and Cephalopoda increase, though not uniformly, upwards to the sixth period. The seventh period is everywhere marked by a zone of sterility, the local extinction of most of the classes, and the introduction of a new order of sediments, brought by a new set of watery currents. Arranging the classes according to their priority of appearance, including Fishes, and giving to each in the successive strata a space proportioned to the number of species, we construct the scheme of proportionate life for the Lower Palæozoic Strata, represented in Fig. 5. |