GEOLOGICAL DISTRIBUTION. The accompanying table indicates in a rough way the larger classi fication of the Crustacea and the distribution and expansion of the various subdivisions in past geologic times. It shows that certain types, es pecially the more primitive, played more impor tant rides in the early ages than they do at pres ent, and that they have been superseded by the more specialized types. The letters at the heads of the vertical columns indicate the geological systems as follows: C', Cambrian; Ordovi Man: S. Silurian: D. Devonian: Cb. Carbonifer ous; P. Permian: T, Triassic: J. Jurassic: Cr. Cretaceous; E, Eocene; Al, 'Miocene: Pl. Plio cene; R. Recent. An interrogation-point indi cates the doubtful presence of a member of the group, and the width of the black line indicates in a very imperfect way the relative amount of expansion of the group. The articles on the dif ferent orders and suborders should he consulted for further information.
ThumoonArny. The best general introduction to the study of the Crustacea is T. H. Huxley, The Crayfish: An Introduction to the Study of Zoology, in International Scientific Series (New York. 1SS4). Other works of a general
nature on the morphology and classification are: J. S. Kingsley, "The of the Arthro poda," in .1 Naturalist, vol. xxviii. (Philadelphia, 1S94) ; K. Groben. "The Geneal ogy and Classification of the Crustacea." in _la nais and Magazine of Natural History, ser. 6, vol. xi. (London. 1S93) : T. R. R. Stebbins. His tory of the Crustarea (New York, 1S93) : R. Rathbun, "Natural History of the Economic Crustaceans," in Bulletin U. S. Fish Commis sion (Washington, ISS91 H. Milne-Edwards, Ilistoire naturellc des cru•taa.s (3 vols.. Paris, 1834-40) ; IL Gerstaecker, "Gliederfiissler." in Brown's Ii lessen and Ordnungen des Thiorreiche, vol. v. ( Leipzig, 1800-94). For description of the fossil forms, consult the various manuals of paleontology, such as those by Zittel, Zittel-East man, Bernard, Nicholson, and Etheridge. A good synopsis of the more especially on the fossil forms, is to be found in Zittcl-East num, Text-Book of Palcontolo?y (New York and London. where are given very useful bib liographie lists of works on both recent and fos sil forms.