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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 nostra, 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. Annelida......... 5
7 Crustacea 16
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 Aviculaceae.
The two Dimyarians are Ctenodonta lævis and Cucullella Anglica (of the family Arcadæ).
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.........
Zoophyta .... 11 *Echinodermata.. 8
3 species. 23 20
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 Arcadæ 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.
2 species. 26
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,
18 Heteropoda.... 1
22 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
.5 Amorphozoa (none)