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Yet the northern character of the Norwich Crag is not fully shown by simply saying that it contains twelve northern species. It is the predominance of certain genera and species, such as Tellina calcarea, Astarte borealis, Scalaria groenlandica, and Fusus carinatus, which satisfies the mind of a conchologist as to the arctic character of the Norwich Crag.
The incumbent till or boulder clay is about 40 feet thick, but it often attains much greater thickness in the same part of Scotland. Pecten islandicus, Moll. Natica clausa, Bred. Trophon clathratum, Linne. Leda truncata. a. Exterior of left valve. b. Tellina calcarea, Chem. Outside of left valve. b.
The presence of these northern shells cannot be explained away by supposing that they were inhabitants of the deep parts of the sea; for some of them, such as Tellina calcarea and Astarte borealis, occur plentifully, and sometimes, with the valves united by their ligament, in company with other littoral shells, such as Mya arenaria and Littorina rudis, and evidently not thrown up from deep water.
We frequently find in it much calcareous spar, or the terra calcarea aerata, which had been in a melted state by heat, and had been crystallized by congelation into a sparry form.
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