Note AB — Note I. Page 13. Shells in Chalk.
Reading edition
Trigonia: a genus of bivalves, of which there are many extinct species in the chalk and oolite; some bands of Portland stone are an aggregation of Trigoniæ: a few very small species, inhabitants of the seas of Australia and New Zealand, are the only known living forms of this once prevailing type of mollusca. See 'Medals of Creation,' p. 407.