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The brachiopoda of the Llandeilo flags, which number 47 species, are in the main the same as those of the Caradoc Sandstone, but the other mollusca are in great part of different species. Orthoceras duplex, Wahlenberg. Russia and Sweden. Asaphus tyrannus, Murchison. Ogygia Buchii, Burm. Syn. Asaphus Buchii, Brongn.
The cephalaspis may be considered as making the smallest advance from the crustacean character; it very much resembles in form the asaphus of lower formations, having a longish tail-like body inserted within the cusp of a large crescent-shaped head, somewhat like a saddler's cutting-knife.
One important remark has been made, that a comparatively small variety of species is found in the older rocks, although of some particular ones the remains are very abundant; as, for instance, of a species of asaphus, which is found between the laminae of some of the slate rocks of Wales, and the corresponding rocks of Normandy and Germany in enormous quantities.
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