. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 414: PAL^ONTOLOGICAL RELATIONS. [Ch. XX, but little information in regard to the condition of marine life, hav- ing yielded at present only 22 mollusca, as before mentioned (p. 412). While, therefore, the break between two great members of the Lower Oolite is expressed by saying that the proportion of species in common only amounts to 12 per cent., we have seen that there is a connection of 24 per cent, between the Upper and Middle, and 21 per cent, be- tween the Mid
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 414: PAL^ONTOLOGICAL RELATIONS. [Ch. XX, but little information in regard to the condition of marine life, hav- ing yielded at present only 22 mollusca, as before mentioned (p. 412). While, therefore, the break between two great members of the Lower Oolite is expressed by saying that the proportion of species in common only amounts to 12 per cent., we have seen that there is a connection of 24 per cent, between the Upper and Middle, and 21 per cent, be- tween the Middle and Lower Oolite; hi other words, there is twice as great a connection between our larger divisions as between two separate members of one of them. * In illustration of shells having a great vertical range, it may be stated that in England 4 species, and 4 only, are known to pass up from the Lower to the Upper Oolite, namely, Rhynchonella obsoleta, Lithodomus inclusus, PJioladomya ovalis, and Trigonia elongata. • Of all the Jurassic Ammonites of Great Fig. 434 Britain, A. Macrocephalus, Schloth, which is common to the Great Oolite and Oxford Clay, has the widest range. That most of the sudden changes of species were due to migration, may be inferred, as Prof. Kamsay remarks, from the fact that, after disappearing from an intermediate formation, Ammonites macrocephalus, tney often reappear in a higher one. But the Schioth. | nat. size. phenomena, on the whole, indicate a constant Great Oolite and Oxford -> . , £ .. ,. -, Clay# dying out ot preexisting species and a coming in of new ones. We have every reason to con- clude that the gaps which occur, both between the larger and smaller sections of the English Oolites, imply intervals of time, elsewhere represented by fossiliferous strata, although no deposit may have taken place in the British area. This conclusion is warranted by the partial extent of many of the minor and some of the larger divisions even in England. &
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