Untitled We cannot leave the last period of the PalAozoic era and its prolonged ice age without noticing that it meant the entire cessation of a large number of ancient types, especially among plants and backboneless animals, which now disappear for ever. It is necessary to understand that the animals of ancient days stand in three different relations to those of to-day. (_a_) There are ancient types that have living representatives, sometimes few and sometimes many, sometimes much changed and sometimes but slightly changed. The lamp-shell, _Lingulella_, of the Cambrian and Ordovician period has a very near relative in the _Lingula_ of to-day. There are a few extremely conservative animals. (_b_) There are ancient types which have no living representatives, except in the guise of transformed descendants, as the King-crab (_Limulus_) may be said to be a transformed descendant of the otherwise quite extinct race to which Eurypterids or Sea-scorpions belonged. (_c_) There are altogether extinct types--_lost races_--which
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