A HISTORY OF ESSEX water in some places for a mile in width ; but my more intimate know- ledge of the upper part is confined to near Mistley, where I have often collected and dredged, though it is too far up to be satisfactory. It will thus be seen that though I have fairly well studied some districts there are large tracts which I have never examined, on account of being too far from any convenient anchorage. It must also be borne in mind that my account of the marine invertebrata of Essex has been made much less complete than it would have been owing to difficulties connected with the oyster fisheries. One cannot dredge in some most suitable and convenient places, and in some cannot even use a small light trawl. In some no one is, or was, allowed to collect on the shore at low water, or to dredge even where there are no oysters. If special facilities had not been conceded to me this article could not have been written. The total length of the coast, including the chief estuaries and creeks, is considerably over 200 miles, and it would take a long time to examine properly the whole. Some of the animals found are so rare and so local that possibly many more species occur than I have been able to collect, though there may not be so much variation along the exposed coasts as in the estuaries. These play a most important part, since the total length of their shores is fully three times that of the more open coast. In summer the water is not much less salt than in the sea, and in some special cases even more salt. The extremes of temperature are also great, since there is relatively such a large extent of very shallow water and of mud banks left dry when the tide is low. In summer in the Orwell I have known it as high as 74, and in winter some of the estuaries are occasionally filled with masses of ice. The conditions are therefore very unsuitable for certain kinds of animals, and they probably explain why some common species are of smaller size than elsewhere. At the same time there is a tendency for the colder water to sink and pass out to sea, and for the warmer to remain in the estuaries and creeks, and therefore in the long run for these to have a higher temperature than the more open water. This is probably one great reason why some parts of the Essex district are so admirably fitted for oyster culture, since the proper development of spat depends so much on a sufficiently high temperature. Along the coast of Essex there are thus no quiet deep water, and no rocks, and no clear rock-pools, but strong currents and great stretches of gravel, sand and mud ; so that the animals found are necessarily of a restricted character. At the same time these circumstances seem favour- able for certain kinds, and some are common which may be very rare in other districts. The coast of Essex is thus peculiar, and estuarine conditions must have great influence as compared with the coast of Norfolk. The following table shows the number of species of such groups of animals as seem to have attracted a fair amount of attention in both counties. Those of Norfolk are taken from a proof of a portion of the Victoria History lent to me by Mr. Walter Garstang. This did not include the 72
