paleozoic rocks, both because they are the most ancient internat forms of organized beings of which we have any knowl

edge, and are very unlike the present races. Representa

tives of all the classes of the animal kingdom are found in what them. Of the vertebrata, fishes-Placoids and Ganoids

only have been discovered.

Trilobites are the principal representatives of the Crus

tacea, which are found in some localities heaped together b in vast multitudes. These animals were of an oval figure,

Fig. 92.

protected by a shield, cov-
ering the anterior parts of
the body, while the ab-
dominal portions consisted
of segments, which ena-
bled them to roll them-
selves into balls, so as to
present their hard crusts
in all directions in self-
defense, as do the lobster
and woodlouse of the pre-
sent day. Their organs
of locomotion, if any exist-
ed, must have been small,
membranaceous or rudi-
mentary, since no distinct
traces of them have yet
been satisfactorily made
out. They were destitute
of antennæ. The back
presents two longitudinal furrows, dividing its surface
into three lobes; hence the name Trilobite. Their eyes
which are in many specimens perfectly preserved, giv-

Trilobite.

ing us some indications of the condition of the early seas in which they lived, were compound, consisting of a great number of angular fagettes or lenses, similar to the eye of the dragon fly, in which there are twenty-five thousand; and of the house fly, in which there are fourteen thousand facettes: the trilobite had several hundreds.

Fig. 93.

In

order to extend the field of vision, since the eye was immovable, it was elevated above their bodies. The form of the eye was that of a frustum of a cone, incomplete on the side opposite the corresponding part of the other eye, enabling the animal to see in all directions horizontally. This structure of the eye indicates that light bore the same relations to vision then, as it now does, and that these animals lived in water sufficiently transparent to transmit light. "We find in these animals," says Dr. Buckland, 66 an optical instrument of most curious construction, adapted to produce vision Eye of the Trilobite. of a particular kind, created in the fulness of perfection, and fitted for the uses and condition of the class of creatures to which this kind of eye ever has been and still is appropriate." Trilobites vary from one to twenty inches in length. They were very abundant among the earliest inhabitants of the globe, and conduration tinued into the carboniferous period, after which no trace of them is discerned. Two hundred and thirty species, under several genera of the family Trilobite, have been

230.5p

Fig. 94.

Calymene.

described; of which the genus Calymene, fig. 94, is found in the Wenlock division of the Upper Silurian rocks.

195. Of the Mollusca, the most numerous representa

Bracheop

Churach

tives in the Silurian rocks belonged to the class Cephalo- 2 ordin poda (§ 147) including the Ammonite and Nautilus, and to the order Brachiopoda of the class Acephala. The latter inhabited bivalve shells, the valves of which, however, were not secured by a hinge, but were confined by a bundle of muscular fibres attached to one shell and passing through an aperture in the beak of the other; this gives to the interior of the shell a peculiar appearance. Several hundred species of the Brachiopoda have been discovered in these rocks. Among them was the genus Terebratula; created among the first, it has survived longer than any others, having several living species in the seas of the present day. The genera Leptæna (Producta,) Delthyris, Orthis (Spirifer,) and Pentamerus, belong to this period.

The Orthoceratite (orthos, straight; keras, horn,) was a threa straight shell, divided into numerous chambers by septa or

partitions, which were perforated so that a tube (siphuncle) might communicate with all of them.

Fig. 95.

Orthoceratite.

196. Of the Radiata of the Silurian period, the coral polyps were the most numerous; their fossils form a very large proportion of the whole mass of the limestones, crowded together as are the corals of coral reefs in the existing The Chain-coral-Catenipora—is a beautiful ge

oceans.

nus characteristic of the Upper Silurian beds. In the lowest beds, and among the earliest forms of life, are found great numbers of a fossil, called, from their markings upon the rocks, Graptolites, supposed to have been zoophytes, Fig. 96.

Graptolithus Ludensis.

Fig. 97.

allied to the sea-pens which inhabit the muddy sediment of the deep waters of the existing oceans. The Graptolites are not found in the limestones, but in the flags or slates of the Cambrian and Silurian rocks, and especially in a mud-stone, which indicates tranquil deposition. The Crinoid or Encrinite family is represented in this group of rocks, but was more fully developed at a later period. The fossils of the Silurian rocks indicate that they were deposited in deep seas, and in a temperature at least as high as that of the tropical regions of the present period. Marine plants were abundant, but the existence of land plants or animals at that period has not been established.

Graptolithus Murchisoni.

197. The igneous rocks associated with the Cambrian and Silurian systems, are granite, porphyry, and greenstone, whose incursions have often given to the strata inclined and contorted positions. But the Silurian beds of Sweden are horizontal, and those of the State of New York are but slightly inclined.

THE OLD RED SANDSTONE-DEVONIAN SYSTEM.

198. This system, consisting of conglomerates, sand- consilly stones, marls, and limestones, is intermediate in all respects between the Silurian and Carboniferous systems. The prevalent color of the marls and sandstones-dull red-is due to the peroxide of iron. It is called the Old Red Sandstone, to distinguish it from the New, which lies above the coal. It is extensively developed in Herefordshire and Devonshire in England, and in Scotland, where it has been fully investigated.* Although the beds of Devonshire differ from those of Herefordshire and Scotland, in mineral and mechanical condition, the characters of their fossils clearly show them to be contemporaneous with the Old Red Sandstone of the other districts; hence the system is sometimes called Devonian.

199. Tabular arrangement of the English and Scottish members of the series:

OLD RED SANDSTONE-ABOUT TEN THOUSAND FEET THICK.

wh

In Scotland the base of the system is an extensive, thick

* "A New Walk in an Old Field-The Old Red Sandstone, by Hugh Miller, Esq.," is a work by a man of self-taught genius, combining the accuracy of scientific research with the fascinations of romance.