The different kinds of mica vary from perfectly colourless and transparent - as in muscovite - through shades of yellow, green, red and brown to black and opaque - as in lepidomelane; the former have a pearly lustre and the latter a submetallic lustre on the cleavage surfaces.
Owen not only occupied himself with the dissection of rare animals, such as the Pearly Nautilus, Lingula, Limulus, Protopterus, Apteryx, &c., and with the description and reconstruction of extinct reptiles, birds and mammals - following the Cuvierian tradition - but gave precision and currency to the morphological doctrines which had taken their rise in the beginning of the century by the introduction of two terms, " homology " and " analogy," which were defined so as to express two different kinds of agreement in animal structures, which, owing to the want of such " counters of thought," had been hitherto continually confused.
In England, Owen's anatomy of the pearly nautilus,14 Huxley's discussion of the general morphology of the Mollusca,17 and Lankester's embryological investigations, 19 have aided in advancing our knowledge of the group. Two remarkable works of a systematic character dealing with the Mollusca deserve mention here - the Manual of the Mollusca, by Dr S.
The Pearly Pool, in a.
Rend., 188 4, 9 8, p. 1 44) obtained a form which he termed nacre (or pearly) sulphur; the same modification was obtained by Sabatier (ibid., 1885, 100, p. 1346) on shaking hydrogen persulphide with alcohol or ether.