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Zoophyta
Echinodermata
Annelida....
Crustacea
Polyzoa
Brachiopoda
Monomyaria
Dimyaria
Gasteropoda
Heteropoda
Pteropoda
Cephalopoda
! Fishes

2

......

30 17 41 27

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4 4 30 7

5 5

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. Brachiopoda... 1

1
Gasteropoda...

1
! Fishes
5

7

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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. SUCCESSIVE SYSTEMS OF MARINE INVERTEBRAL

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 known.

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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

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 Paleozoic Strata, represented in Fig. 5.

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LIFE.

The system of life thus constituted in the seas of the most ancient period so far resembles the system now established in modern oceans as to contain the same classes with similar functions and dependencies. But there are great differences in the relative proportions of the classes, and of the tribes which are included in them. And these differences are to a certain extent dependent on the elapsed time; classes at first very small have grown very large, others once predominant have been greatly diminished. To make this evident it will be useful to give up the mere enumeration of species in each class, and to adopt as a basis of representation the proportions in which the classes stand to each other in each period.

This can be easily done by equating the sum of the species in each period to 1000, and the number of species in each class to its proportion of the whole. Choosing for this purpose eight principal classes or assemblages, and tracing their relative proportions in successive periods, we arrive at the result represented in the parallelogram, Fig. 6.

In this representation of the relative proportions of the several classes in successive geological periods,

R. L.

G

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