In certain algae of different families the thallus is encrusted with carbonate of lime, and is thus rendered much more resistant. The Diatoms, on the other hand, possess still more durable siliceous tests which are particularly well adapted to resist the solvent action of water and other agents of destruction. It is these calcareous and siliceous forms which supply the greater part of the trustworthy data furnished by fossil algae. [Sidenote: SCARCITY OF FOSSIL ALGAE.] It remains to consider some of the causes to which we may attribute the scarcity of fossil algae, and the possible sources of error which beset any attempt to describe or assign names to impressions and casts simulating algal forms. In the first place, the delicate nature of algal cells is a serious obstacle to fossilisation. Even in plants in which the woody stems have been preserved by a siliceous or calcareous solution, we frequently find the more delicate cells represented by a mass of crystalline matter without any trace of the cell-walls being preserved. In such plants as algae, where the cell-walls are not lignified, but consist of cellulose or some special form of cellulose, which readily breaks down into a mucilaginous product, the tissues have but a small chance of withstanding the wear and tear of fossilisation. The danger of relying on external form as a means of recognition is especially patent in the case of those numerous markings or impressions frequently met with on rocks, and which resemble in outline the thallus of recent algae. Among animals, such as certain Polyzoa, the flat branching body of various algae is closely simulated, and in other plants, such as the frondose liverworts, the same thalloid and branched form of body is again met with. Some of the much dissected _Aphlebia_ leaves of ferns (e.g. _Rhacophyllum_ species) bear a striking resemblance to fossil algae; and numerous other examples might be quoted. In palaeobotanical literature we find a host of names, such as _Chondrites_, _Fucoides_[224], _Caulerpites_ and others applied to indefinite and indistinct surface markings which happen to resemble in shape certain of the better known genera of recent seaweeds.