Kr., _Idothea entomon_ Lin., _Idothea Sabinei_ Kr., two species of Lysianassida, _Pontoporeia setosa_ Stbrg., _Halimedon brevicalcar_ Goës, an Annelid, a Molgula, _Yoldia intermedia_ M. Sars, _Yoldia_ (?) _arctica_ Gray, and a Solecurtus[39]. "The temperatures were taken by a centigrade thermometer. Again we read of the results of dredging off Cape Chelyuskin. "The yield of the trawling was extraordinarily abundant; large asterids, crinoids, sponges, holothuria, a gigantic sea-spider (Pycnogonid), masses of worms, crustacea, etc. _It was the most abundant yield that the trawl-net at any one time brought up during the whole of our voyage round the coast of Asia_, and this from the sea off the northern extremity of that continent[40]." [Footnote 38: For a discussion as to the value of plants as indices of climate see Seward, A. C., Sedgwick Essay for 1892.] [Footnote 39: Nordenskjöld, A. E., _The Voyage of the Vega_, Vol. I. Chap. IV.] [Footnote 40: _Ibid._ Chap. VII.] Amongst the marine invertebrates reef-building corals and mollusca perhaps furnish the best evidence of climatic conditions. The coral-reefs of the Jurassic rocks with large gastropods and lamellibranchs clustered around them have been appealed to in proof of the existence of sub-tropical conditions during their formation; further back in time we find evidence of climate furnished by the fossils of the Silurian rocks of the Isle of Gothland in the Baltic Sea. Of these, Lindström writes "_The fauna had a tropical character_. In consideration of the great numbers of Pleurotomariae, Trochi, Turbinidae and the large Pteropods the assumption of a tropical character of the fauna may seem justifiable[41]." [Footnote 41: Lindström, G., _On the Silurian Gastropoda and Pteropoda of Gotland_, Stockholm, 1884, p. 33.] Structure may give some indication of climate even though the organism is not allied to living species. The bark of trees in arctic regions is often thicker than in more temperate regions, and the leaves of arctic plants often have special characters to enable them to resist the long periods during which they are deprived of water, though the fact that desert-plants frequently shew similar modifications deprives this test of any particular value except as a means of corroborating conclusions reached from other evidence[42]. The shells of arctic mollusca may become stunted, but this is not by any means universal, and the same result may be brought about by other abnormal conditions, as for instance the increase of salt in a water area by evaporation. [Footnote 42: For an account of the modifications of the leaves of arctic plants, see Warming, Eug., _Om Grønlands Vegetation_, Meddelelser om Grønland, 12th part, p. 105.]