Vertebrata of the Eocene of Wyoming”—­’Geological
     Survey of Montana,’ &c., 1872.  Cope.
(48) “Ancient Fauna of Nebraska”—­’Smithsonian Contributions to
     Knowledge,’ vol. vi.  Leidy.
(49) ‘Manual of Geology.’  Dana. (50) “Palaeontology and Evolution” (Presidential Address to the
     Geological Society of London, 1870)—­’Quart.  Journ.  Geol. 
     Soc.,’ vol. xxvi.  Huxley.’
(51) ‘Mineral Conchology.’  Sowerby. (52) ‘Description des Coquilles Fossiles,’ &c.  Deshayes. (53) ‘Description des Coquilles Tertiaires de Belgique.’  Nyst. (54) ‘Fossilen Polypen des Wiener Tertiaer-beckens.’  Reuss. (55) ’Palaeontologische Studien ueber die aelteren Tertiaer-schichten
     der Alpen.’  Reuss.
(56) ‘Land und Suess-wasser Conchylien der Vorwelt.’  Sandberger. (57) ‘Flora Tertiaria Helvetica.’  Heer. (58) ‘Flora Fossilis Arctica.’  Heer.
(59) ’Recherches sur le Climat et la Vegetation du Pays
     Tertiaire.’  Heer.
(60) ‘Fossil Flora of Great Britain.’  Lindley and Hutton. (61) ‘Fossil Fruits and Seeds of the London Clay.’  Bowerbank. (62) “Tertiary Leaf-beds of the Isle of Mull”—­’Quart.  Journ. 
     Geol.  Soc.,’ vol. vii.  Edward Forbes.
(63) ‘The Geology of England and Wales.’  Horace B. Woodward.[25]

[Footnote 25:  This work—­published whilst these sheets were going through the press—­gives to the student a detailed view of all the strata of England and Wales, with their various sub-divisions, from the base of the Palaeozoic to the top of the Tertiary.]

CHAPTER XXI.

THE QUATERNARY PERIOD.

THE POST-PLIOCENE PERIOD.

Later than any of the Tertiary formations are various detached and more or less superficial accumulations, which are generally spoken of as the Post-Tertiary formations, in accordance with the nomenclature of Sir Charles Lyell—­or as the Quaternary formations, in accordance with the general usage of Continental geologists.  In all these formations we meet with no Mollusca except such as are now alive—­with the partial and very limited exception of some of the oldest deposits of this period, in which a few of the shells occasionally belong to species not known to be in existence at the present day.  Whilst the Shell-fish of the Quaternary deposits are, generally speaking, identical with existing forms, the Mammals are sometimes referable to living, sometimes to extinct species.  In accordance with this, the Quaternary formations are divided into two groups:  (1) The Post-Pliocene, in which the shells are almost invariably referable to existing species, but some of the Mammals are extinct; and (2) the Recent, in which the shells and the Mammals alike belong to existing species.  The Post-Pliocene deposits are often spoken of as the Pleistocene formations (Gr. pleistos, most; kainos, new or recent), in allusion to the fact that the great majority of the living beings of this period belong to the species characteristic of the “new” or Recent period.