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Jurassic Period.

Having so limited a total range in time, but including some hundreds of well-marked specific forms, and being widely diffused in geographical space, Ammonites furnish to the Paleontologist admirable data for fixing the chronological succession of the secondary strata, in cases when these strata are not continuously traceable. Taking some well-known forms, including Ceratites, and placing them in the order of their superposition, we obtain the following series of geological epochs in the great Mesozoic Period.

No Ammonites or Ceratites or Goniatites in Cenozoic Strata.

[blocks in formation]
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A. Parkinsoni............Inf. Oolite upper part.

A. Humphreysianus...Inf. Oolite middle part.
A. Murchisonæ.........Inf. Oolite lower part.

A. Jurensis..
A. bifrons

[blocks in formation]

..Sand.
..Upper Lias.
.Lias Limestone.

.Lowest Lias.

.Muschelkalk.

No Ammonites or Ceratites in the Paleozoic Strata.

Fishes commencing, as already stated, in the upper Silurian Strata, become from that point highly important in geological history—more important than even their numerous remains at first seem to indicate; for though teeth, scales, and fin-rays of these animals. are not scarce in the strata, these, the most conservable of the hard parts of fishes, are scattered irregularly, and, until studied after the method of Agassiz, give but slight information. Under the hands of this great naturalist and his disciples, Egerton and Enniskillen, we have seen the history of fossil fishes grow to embrace many hundreds of distinct forms, very interesting in physiology, always very valuable in geological reasoning. The method of Agassiz is no doubt in some degree conventional, and specially framed for the study of fossils, yet the characters derived from the dermal covering are always of high value in the classification of the vertebrata, and specially influential in fishes.

Two great orders of fishes have enamelled scales e. g. Placoid and Ganoid Fishes-the latter have enamel externally, bone internally for each scale, and the scales so closely packed as to constitute a real dermo-skeleton. These orders occur in all the strata above the Silurians, and still exist: they are the only orders which occur in the Paleozoic rocks; in the existing ocean and in the Tertiary Strata they are comparatively the least abundant.

Two other orders have horny scales, neither bony nor enamelled-viz. Cycloid and Ctenoid Fishes; and these, beginning their courses with the Mesozoic Strata, constitute by far the largest portion of the existing race of fishes. Here again we perceive the strong contrast between the Paleozoic and Neozoic inhabitants.

Existing marine Fishes have tails formed on different models, as the forked tail of the Salmon, the rounded tail of the Wrasse, the elongate pointed tail of the Conger, and the unequally lobed tail of the Shark. The three former tails may be called regular or symmetrical, the latter unsymmetrical; or if we please, the fishes possessing the former may be called Homocercal, while the latter may be called Heterocercal. In existing nature the Homocercal fishes are by far the most numerous; they are plentiful in Cenozoic and Mesozoic Strata, but they are unknown in the Paleozoic rocks, where Heterocercal fishes alone occur. This is the more remarkable because both in the Paleozoic and Mesozoic rocks Ganoid fishes abound; still Palæozoic races are all heterocercal, Mesozoic are mostly homocercal. In all periods Placoid fishes were heterocercal.

The group of Placoid Fishes is the only one which has existed through all geological periods to the present day; it includes a large range and gra

dation of structure, so that while some races may probably be regarded as of the highest type in the class, others (if we include Myxine) can only just claim to be vertebrata. The fossil races belong generally to the higher types. The skeleton being cartilaginous or mainly so, it often happens that the vertebral column is not preserved: indeed very frequently only a few scattered teeth and fin-rays remain to attest the existence and magnitude of these ancient mostly shark-like creatures. Singular to say, one group of these fishes, represented in a living state by one Australian species (Cestracion Philippi), has been traced through all these periods (except Canozoic) by its teeth and fin-rays, abounding in Palæozoic and Mesozoic ages; while, on the other hand, the ordinary sharks of the modern seas are represented in the Tertiary strata plentifully, in the Mesozoic sparingly, in the Paleozoic not at all. The Cestraciontidæ have, besides pointed teeth in front, some very large and broad behind, not pointed but suited for crushing and grinding hard and solid substances, such as shell-covered Mollusca, Crustacea, Echinodermata, or even Ganoid fishes1.

Reptilia according to Owen' may be arranged in thirteen orders, of which five, viz. 1. Batrachia (Frogs),

1 Buckland, Br. Tr. Pl. 41.

2 Reports of British Association, 1859.

II. Chelonida (Turtles), III. Ophidia (Snakes), Iv. Lacertilia (Lizards), and v. Crocodilia (Crocodiles), are both recent and fossil; and eight, viz. vI. Deinosauria, VII. Thecodontia, VIII. Pterosauria, IX. Anomodontia, x. Sauropterygia, x1. Ichthyopterygia, XII. Labyrinthodontia, XIII. Ganocephala are only found in a fossil state. Among the marine tribes are some of the Chelonida, all the fossil Ichthyopterygia, Sauropterygia, some of the Crocodilia and Lacertilia, possibly the Thecodontia1, omitting the Chelonida. They stand in the following order of time:

Mososaurus...

Leiodon

Pleiosaurus.

Cetiosaurus...

.Chalk.

.Chalk.

Kimmeridge Clay.

..Great Oolite and upwards to Wealden.

Teleosaurus... ........Lias and upwards to Chalk.

Ichthyosaurus.. ...........Lias and upwards to Chalk.

Plesiosaurus Bone Bed, Aust, and upwards to Chalk. Stagonolepis ...... Triassic Sandstone of Elgin, Thecodontosaurus. Conglomerate, Bristol.

It will be seen hereafter that Terrestrial and Freshwater Reptilia appear to be of higher antiquity than the marine tribes yet discovered. Remains of Chelonida appear to be about of the same antiquity as the marine Saurians; they occur in the Red

1 The fossil Crocodilia may perhaps have visited rivers and the land; their legs might allow of this, and their feet are not unsuitable.

2 Huxley, Geol. Soc. Journal, 1859.

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