Could we obtain a deep section of the bottom, we should find beds of Ostrea edulis and Cardium edule, then a layer of Zostera marina with fresh-water fish, and then a bed of Mytilus edulis.

The great oyster is here numerous in layers; the Trigonocelia and Turritella are also very numerous: it is remarkable that the Pecten Paranensis, so common in all other parts of the coast, is here absent: the shells consist of: Ostrea Patagonica, d'Orbigny; "Voyage Pal." Venus meridionalis of G.B. Sowerby. 4. Crassatella Lyellii, G.B. Sowerby. 5. Cardium puelchum, G.B. Sowerby. 6.

On this there are two strata of the pale brown mudstone, also like that of S. Josef, separated by a darker-coloured, more argillaceous variety, including the Ostrea Patagonica. Professor Ehrenberg has examined this mudstone for me: he finds in it three already known microscopic organisms, enveloped in a fine-grained pumiceous tuff, which I shall have immediately to describe in detail.

The limestone, where purest, is white, highly crystalline, and full of cavities: it includes small pebbles of quartz, broken shells, teeth of sharks, and sometimes, as I was informed, large bones: it often contains so much sand as to pass into a calcareous sandstone, and in such parts the great Ostrea Patagonica chiefly abounds.

In one of these cliffs, sixty feet above the sea, beds of mussels were found: ostrea, pinna, chama; according to Dr. V. M. O. denticula, Bron.; O. cornucopiae, Chemn.; O. rosacea, Desh.; Chama sulfurea, Reeve; Pinna Nigrina, Lam.

These strata may be seen in the Isle of Wight in contact with the chalk, or in the London basin, at Reading, Blackheath, and Woolwich. In some of the lowest of them, banks of oysters are observed, consisting of Ostrea bellovacina, so common in France in the same relative position. In these beds at Bromley, Dr.

Among the mollusks, a remarkable ribbed Melania, of the section Chilina, occurs. Ostrea distorta, Sowerby. Cinder-bed. Hemicidaris Purbeckensis, E. Forbes. Cyprides from the Middle Purbecks. a. Cypris striato-punctata, E. Forbes. b. Cypris fasciculata, E. Forbes. c. Physa Bristovii, E. Forbes. It was accompanied by a species of Perna.

M. Domeyko has sent to France a collection of fossils, which, I presume, from the description given, must have come from the neighbourhood of Arqueros; they consist of: Pecten Dufreynoyi, d'Orbigny, "Voyage" Part Pal. Ostrea hemispherica, d'Orbigny, "Voyage" Part Pal. Turritella Andii, d'Orbigny, "Voyage" Part Pal. Hippurites Chilensis, d'Orbigny, "Voyage" Part Pal.

The remarks given on the several foregoing shells, show that, in M. d'Orbigny's opinion, the Pecten, Ostrea, Turritella, and Hippurite indicate the cretaceous period; and the Gryphaea appears to Professor Forbes to be identical with a species, associated in Southern India with unquestionably cretaceous forms.

Cardium dissimile. 1/4 natural size. Ostrea expansa. It supplies the well-known building-stone of which St. Paul's and so many of the principal edifices of London are constructed. About fifty species of mollusca occur in this formation, among which are some ammonites of large size.