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Crustacea

7 genera.

Polyzoa

One genus.
Brachiopoda 2 genera.

Agnostus-ConocephalusEllipsocephalus - Hymenocaris - Olenus - Paradoxides - Protichniites

11 species. Dictyonema-1 species. Lingula-Orthis. 3 species.

The list is taken from a valuable catalogue of the Lower Palæozoic Fossils of the British Isles, in Murchison's Siluria, Ed. II. 18591. The Crustacea belong mostly to the great family of Trilobitidæ. Norway and North America yield each a series of the same general character. A larger number of species has been detected by M. Barrande in Bohemia, with the same general aspect. Individuals of the Oleni, Lingulæ and Oldhamiæ are abundant in certain layers, but a large portion of the strata of this 'Primordial' Zone is devoid of all traces of life.

Small as is this series of generic forms, we recognize representatives of each of the three great branches of the Invertebrata-there are carnivorous feeders as well as creatures nourished by cellular or vegetable food and Fucoids are traced both in Norway and Malvern, in beds below the Oleni. In

1 Three species of Oleni found by myself in the black shales of Malvern are included in this zone, though in the Catalogue referred to they are placed in the Caradoc formation. Murchison, however, assigns these black shales to the early date here adopted for them. Siluria, 45 et seq.

what sense a system of life restricted to these few and mostly small elements can be regarded as the ancestral type of the races now living in the sea,

gentis cunabula nostræ,

will perhaps appear as we proceed. Lingula is henceforward always present in every great group of strata till we reach the Tertiaries.

The next combination which attracts attention occurs in the superincumbent group of strata, called the Llandeilo formation. The following summary will be sufficient for our purpose:

Zoophyta......... 4 genera......... 4 species.

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The classes of Invertebrata are now augmented to eleven: the new classes, all Molluscous, marked by the asterisk, being very poor in species, while the previously existing classes have become much more fertile. The Annelida include both Cephalobranchiate and Dorsibranchiate tribes.

The Crustacea are nearly all Trilobites.

Among the Brachiopoda is no Rhynchonella or

Terebratula.

The single Monomyarian is Ambonychia Triton, from Bird-hill, near Llandeilo, of the family Aviculaceæ.

The two Dimyarians are Ctenodonta lævis and Cucullella Anglica (of the family Arcade).

These two families stand near to one another in the methods of naturalists.

Among the characters of this period is the relative prevalence of Pteropod and Polyzoan species, and the absence of Echinodermata, if indeed the Haverfordwest Strata which yield Echinosphærites and Sphæronites, be rightly removed from this formation.

The Carnivorous tribes are augmented by the Cephalopoda, which henceforward are found to sustain a very important part in the economy of the sea, and in a less degree by the Pteropoda and Heteropoda.

A third combination may be examined in the Caradoc formation, which indeed is scarcely to be separated from the Llandeilo series, but is much richer in species.

*Amorphozoa 2 genera.......... 3 species.

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Two new classes, Amorphozoa and Echinodermata, appear among the lower groups of the Invertebrata. Crustacea ascend to a great predominance, both in number, variety, and magnitude, still belonging to the same divisions; Cephalopoda, Gasteropoda and Dimyaria grow to be numerous. Thus the aspect of the marine Fauna is greatly altered.

Brachiopoda abound, and now Rhynchonella, an existing genus, appears, but without Terebratula, which is also living. Among the Dimyaria, Pleurorhynchus is recorded, but Arcade and Mytilidæ are the predominant races, Aviculidæ still the only Monomyarian group. Among the Gasteropoda Murchisonia prevails. Orthoceras, comparatively rare in Llandeilo rocks, is here abundant. Star-fishes appear among Echinodermata.

The series of strata next above is regarded by Murchison as a transitional or intermediate group between the Caradoc and Wenlock formations, and receives from him the name of Llandovery Rocks. Hardly a single genus (Nidulites may perhaps be one exception), but as many as 71 out of 193 species seem to have been found exclusively in these deposits.

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We find in the Upper Silurian Strata the system of life unequally represented in the different parts of the series, most full in the Wenlock and Ludlow groups, and reduced almost to nothing in the uppermost beds. In the Wenlock group, which contains the principal calcareous mass of the Silurian Strata, we find:

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