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 A HISTORY OF NORFOLK ities of the marine zoology of Norfolk waters ; and although a detailed analysis of the Norfolk marine fauna from this point of view would be premature in the present state of knowledge, it is nevertheless probable that, as suggested by Metzger for the southern North Sea fauna in general, the paucity of the shore fauna is largely attributable to the excessive seasonal alterations of temperature, which render the summers too hot for purely northern species and the winters too cold for the per- manent residence of purely southern types. Indirectly this view receives corroboration from the exceptional numbers of migratory species which frequent East Anglian waters in the summer and autumn months, but retire either to north or south with the approach of winter. Thus the herring, an essentially northern fish, does not arrive until the excessive summer heat has begun to wane in September, and retires to deeper water towards the end of November, when a rapid fall of temperature usually takes place off Norfolk and Suffolk. On the other hand, the mackerel, an essentially southern ' fish, invades Norfolk waters from the English Channel in May, when the local temperature has begun to ex- ceed the winter temperature at the mouth of the English Channel ; and the same fish finally retires in October or November, when the temper- ature has begun to fall markedly below the temperature then prevalent in the Channel. The following record of average monthly temperatures off the north and east coasts of Norfolk will enable the student, by careful use of the thermometer, to tell whether in any particular year the sea is warmer or colder than usual, and thus perhaps to trace the causes of any exceptional phenomena which may occur in the migrations or abundance of particu- lar members of the Norfolk fauna. The figures are derived from Mr. H. N. Dickson's valuable paper in the Quart. Journ. Roy. Met. Soc. (vol. xxv., October, 1899), and are computed from the regular observations for eighteen years of men employed on the lightships mentioned. The temperatures taken further inshore on the Newarp light -vessel are practically identical with those recorded from the Leman and Ower light-vessel. The scale is that of Fahrenheit : — Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Outer Dowsing L. V. 416 40-8 410 432 463 52-5 556 57-2 567 532 488 451 Leman and Ower L. V. 409 399 40-5 431 476 531 573 591 579 537 489 443 Another factor, equally important in determining the character of the Norfolk fauna, is the nature of the ground and coast-line. The ex- posed coast, and the lack of sheltered creeks and inlets, render the fauna of Norfolk less rich than that of Essex, although the latter region is sub- ject to temperature changes closely resembling those of Norfolk. The scarcity of rocks also deprives the region of great numbers of fixed and climbing forms which would otherwise be present. The fauna thus consists principally of those sand- or mud-loving types which are capable of withstanding an exceptionally wide range of {Trigla hirundo), which are sometimes caught in large numbers. 78
 * Other examples of southern immigrants in summer are the red mullet, anchovy, and latchet