_Fluvio-marine crag of Norwich._--At several places within five miles of Norwich, on both banks of the Yare, beds of sand, loam, and gravel, provincially termed "crag," occur, in which there is a mixture of marine, land, and freshwater shells, with ichthyolites and bones of mammalia. It is clear that these beds have been accumulated at the bottom of the sea near the mouth of a river. They form patches of variable thickness, resting on white chalk, and are covered by a dense mass of stratified flint gravel. The surface of the chalk is often perforated to the depth of several inches by the _Pholas crispata_, each fossil shell still remaining at the bottom of its cylindrical cavity, now filled up with loose sand which has fallen from the incumbent crag. This species of Pholas still exists and drills the rocks between high and low water on the British coast. The most common shells of these strata, such as _Fusus striatus_, _Turritella terebra_, _Cardium edule_, and _Cyprina islandica_, are now abundant in the British seas; but with them are some extinct species, such as _Nucula Cobboldiae_ (fig. 120.) and _Tellina obliqua_ (fig. 121.). _Natica helicoides_ (fig. 122.) is an example of a species formerly known only as fossil, but which has now been found living in our seas. Among the accompanying bones of mammalia is the _Mastodon_ _angustidens_[149-A] (see fig. 130.), a portion of the upper jawbone with a tooth having been found by Mr. Wigham at Postwick, near Norwich. As this species has also been found in the Red Crag, both at Sutton and at Felixstow, and had hitherto been regarded as characteristic of formations older than the Pleistocene, it may possibly have been washed out of the Red into the Norwich Crag. [Illustration: Fig. 120. _Nucula Cobboldiae._] [Illustration: Fig. 121. _Tellina obliqua._] [Illustration: Fig. 122. _Natica helicoides_, Johnston.] Among the bones, however, respecting the authenticity of which there seems no doubt, may be mentioned those of the elephant, horse, pig, deer, and the jaws and teeth of field mice (fig. 141.). I have seen the tusk of an elephant from Bramerton near Norwich, to which, many serpulae were attached, showing that it had lain for some time at the bottom of the sea of the Norwich Crag.