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 CH. V. other may be found related to geological time. This is so far correct that it is only in and above the chalk, that the recent or true crocodilian type of vertebræ has been recognised. But there is a third and singular modification of fossil saurians; for in streptospondylus, which occurs in lias, oolite, and wealden, the vertebræ are, contrary to those of crocodile, convex before and concave behind. (Cuvier, Ossemens fossiles; Von Meyer, Palæologica; Conybeare, De la Beche, Riley, Stutchbury, in Geol. Transactions; Owen, Brit. Assoc. Reports, 1839, 1841.)

Birds.—The remains of birds are extremely uncommon, even among the comparatively recent alluvial lacustrine and cavern deposits, still less frequent among the tertiary strata, and almost unknown among the older strata. This is one of many instances which agree in proving that the occurrence of the exuviæ of land animals and land plants in the stratified rocks, which were formed chiefly in the sea, is the result of causes so local, limited, and rare, as to be, in fact, accidental, and therefore no sufficient basis of reasoning as to what was the state of the ancient land at particular geological periods. At the present day we could learn little concerning the vegetables and animals of the land, from the few traces which remain of them in the beds of lakes, rivers, and the sea.

Mammalia.—The argument just used may be applied with equal justice to the paucity of remains of land mammalia in the marine strata of all ages; for even in the tertiary rocks such remains are rare. But it is, perhaps, necessary to find other causes for the scarcity of marine mammalia in all except certain of the tertiary strata and superficial sediments. The opinion formerly favoured was, that during the whole of the primary and secondary periods, at least, the class of