the siliceous frustules of Diatoms. In the Michigan nodules the same _Schizothrix_ occurs, associated with _Stigonema_ and _Dichothrix_, other genera of the Nostocaceae. One of the Michigan pebbles is shown in section in fig. 32 _D_. [Sidenote: OOLITIC STRUCTURE.] The connection between the well-known oolitic structure, characteristic of rocks of various ages in all parts of the world, and the presence of algal cells is of the greatest interest from a geological point of view. In recent years considerable attention has been paid to the structure of oolitic rocks, and in many instances there have been found in the calcareous grains tubular structures suggestive of simple cylindrical plants, which have probably been concerned in the deposition of the carbonate of lime of which the granules consist. In 1880 Messrs Nicholson and Etheridge[185] recorded the occurrence of such a tubular structure in calcareous nodules obtained from a rock of Ordovician age in the Girvan district of Scotland. These Authors considered the tubes to be those of some Rhizopod, and proposed to designate the fossil _Girvanella_. _Girvanella_ (fig. 26). Messrs Nicholson and Etheridge defined the genus as follows:— “Microscopic tubuli, with arenaceous or calcareous (?) walls, flexuous or contorted, circular in section, forming loosely compacted masses. The tubes, apparently simple cylinders, without perforations in their sides, and destitute of internal partitions or other structures of a similar kind.” [Illustration: FIG. 26. _Girvanella problematica_, Eth. and Nich. Tubules of _Girvanella_ lying in various positions and surrounding an inorganic ‘nucleus’ or centre. From a section of Wenlock limestone, May Hill. × 65] Since this diagnosis was published very many examples of similar tubular fossils have been described by several writers in rocks from widely separated geological horizons. The accompanying sketch (Fig. 26), drawn from a micro-photograph kindly lent to me by Mr Wethered of Cheltenham, who has made oolitic grains a special subject of careful investigation, affords a good example of the occurrence of such tubular structures in an oolitic grain of Silurian age from the Wenlock limestone of May Hill, Gloucestershire[186]. In the centre is a crystalline core or nucleus round which the tubules have grown, and presumably they had an important share in the deposition of the calcareous substance. The nature of _Girvanella_, and still more its exact position in the organic world, is quite uncertain; it is mentioned rather as _à propos_ of the association of recent Cyanophyceae with oolitic structure, than as a well-defined genus of fossil algae.