The _Coralline Crag_, of very limited extent in this country, ranges over about twenty miles between the rivers Stour and Alde, with a breadth of three or four. It consists of two divisions--an upper one, formed chiefly of the remains of Bryozoa, and a lower one of light-coloured sands, with a profusion of shells. The upper division is about thirty-six feet thick at Sudbourne in Suffolk, where it consists of a series of beds almost entirely composed of comminuted shells and remains of Bryozoa, forming a soft building-stone. The lower division is about forty-seven feet thick at Sutton; making the total thickness of the Coralline Crag about eighty-three feet. Many of the Coralline Crag Mollusca belong to living species; they are supposed to indicate an equable climate free from intense cold--an inference rendered more probable by the prevalence of northern forms of shells, such as _Glycimeris_, _Cyprina_, and _Astarte_. The late Professor Edward Forbes, to whom science is indebted for so many philosophical deductions, points out some remarkable inferences drawn from the fauna of the Pliocene seas.[95] It appears that in the glacial period, which we shall shortly have under consideration, many shells, previously established in the temperate zone, retreated southwards, to avoid an uncongenial climate. The Professor gives a list of fifty which inhabited the British seas while the Coralline and Red Crag were forming, but which are all wanting in the glacial deposits;[96] from which he infers that they migrated at the approach of the glacial period, and returned again northwards, when the temperate climate was restored.[97] [95] Edward Forbes in “Memoirs of the Geological Survey of Great Britain,” vol. i., p. 336. [96] For full information on these deposits the reader is referred to the “Memoirs on the Structure of the Crag-beds of Norfolk and Suffolk,” by J. Prestwich, F.R.S., in the _Quart. Jour. Geol. Soc._, vol. xxvii., pp. 115, 325, and 452 (1871). Also to the many Papers by the Messrs. Searles Wood published in the _Quar. Jour. Geol. Soc._, the _Ann. Nat. Hist._, the _Phil. Mag._, &c. [97] Lyell’s “Elements of Geology,” p. 203. In the Upper or Mammaliferous (or Norwich) Crag, of which there is a good exposure in a pit near the asylum at Thorpe, bones of Mammalia are found with existing species of shells. The greater number of the Mammalian remains have been supposed, until lately, to be extraneous fossils; but they are now considered by Mr. Prestwich as truly contemporaneous. The peculiar mixture of southern forms of life with others of a more northern type lead to the inference that, at this early period, a lowering of temperature began gradually to set in from the period of the Coralline Crag to that of the Forest Bed, which marks the commencement of the Glacial Period.