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PALAEONTOLOGY.

Spondylus — begins in the Chalk.

Ostrea. This well-known genus of our present seas was early represented in theancient faunae under peculiar and distinctive forms: to the Chalk it yielded manyspecies, some of which were very remarkable from the complicated manner in whichthe shell was folded or plaited.

Brachiopoda . —The animals of this great class, which appeared with the very firstof organic beings in the most ancient fauna: of the earth, the Silurian , are less perfectin their organization than those of preceding classes. They have no head, haveneither organs of vision nor of hearing, and are deprived of all organs of motion.Whether free or attached, the species cannot change their place, and must thereforebe peculiarly sensitive of great cosmical changes. It is thus that they have becomepowerful means of distinguishing formations or epochs, as may be judged from thefollowing statement of the range of the several genera, in which it will appear thatthey are even more strikingly limited than in the other classes.

Lingula , from the Silurian up to the recent epoch, where it still exists.

Productus — Chonetes — Leptagonia — Lepteena —from the Silurian to the Triasic inclusive, and then disappear.

Orthis — Orthisina — Strephomena — Hemithiris — Strigocephalus — Porambonites —Rhynchonella, — of these only Rhynchonella is continued to the Chalk, where itceases.

The eight genera constituting the families Uncitidce and Spiriferidm —not continuedto the Chalk.

Magas is peculiar to the Chalk, and Terebratulina commences with it.

Of the family of Terebratulidee, as restricted by D’Orbigny , the genus Terebratulais common to all geological formations, and is still living, whilst Terebratella, Tere-brirostra, and Fissirostra are peculiar to the Chalk.

And of the second division of Brachiopoda , Megathiris and Thecidea appear in theChalk, and are still living, the latter being attached to Coral at great depths.

Of the family of Caprinidce, the remarkable genera Ilippurites, Caprina, Caprinula,and Caprinella are all peculiar to the Chalk, and would, even taken alone, stamp anair of peculiarity on the fauna of the Chalk.

In like manner, the curious genera Radiolides, Biradiolides, Caprotina, and Re-quienia, which constituted the family of Radiolides of D’Orbigny , distinguished theCretaceous fauna, in which, at several successive stages, their several species weregrouped together,— forming in deep water extensive reefs, like the coral-reefs of ourpresent seas. Is it possible, then, to look at such phenomena without at once per-ceiving that the work of the Creator has been complete in each successive fauna; andthat if in one a peculiar object was attained by the instrumentality of certain organisms,that object was equally attained in another, either by the agency of different organicbeings, or by a different combination of the same ?

These great truths would be equally supported, were we to extend the inquiry tothe Echinodermata , and specially to the Echinidee, as in the numerous genera andspecies of the Cretaceous epoch would be found some approximating to our recenttypes, but many (see Geology Plate XIII.) peculiar to the Chalk. Here also theCrinoidte, which, like the cephalopodous Molluscs of the tetrabranchiate order, com-menced with the earliest epochs, and have now almost disappeared or have becomecomparatively a mere shadow; whilst the Echinida; have assumed an importance asto numbers which they still maintain. The fauna of the Chalk, therefore, by turnsapproaches to or recedes from the ancient fauna:, on the one hand, and the existingfauna on the other; but though even a slight specific connection with the recent has