USE OF FOSSILS IN CLASSIFICATION OF STRATA. 527 



animals composing the several faunas. The Sponges give no indication 

 of the larger divisions of time ; the Foraminifera introduce their new 

 types gradually, so that we look to the Carboniferous rocks, the Trias, 

 and the Chalk as furnishing the majority of existing genera. Among 

 corals the Alcyonarians are scantily developed, yet date back to the 

 older Primary rocks. The Rugosa are chiefly, though not exclusively, 

 of Primary age ; the Sclerobasic corals are not known prior to the 

 Tertiary period ; and the Perforata, which are common corals at the 

 present day, date from the Cambrian rocks"; the Aporosa are more 

 numerous in the newer rocks than in the Primary period. The sea- 

 urchins would tend to unite Secondary and Tertiary rocks together, 

 while some urchin types show a remarkable connection between the 

 Cretaceous and Tertiary periods. The Crinoids are an asthenoid group 

 most numerous in the Primary period ; but otherwise have little value 

 in stratigraphical classification. The living groups of Crustacea do not 

 suggest any of the existing divisions of the strata, since the higher 

 forms are chiefly known from the Lias, the Cambridge Greensand, and 

 the London Clay. The Lamellibranchiata furnish many surviving 

 types in the Primary rocks, especially the Carboniferous ; others become 

 known with the Trias, Lias, Neocomian, Cretaceous, and Lower and 

 Middle Tertiary. The Gasteropods commence gradually, one or two' 

 with a formation, though they are most numerous in the Carboniferous, 

 Lias, and Chalk, until the Lower Tertiary introduces the majority of 

 living forms. Hence there are nine or ten great palseontological divi- 

 sions of British strata. 



Paleontology has often been regarded merely as the aid which a 

 naturalist contributes to the work of the stratigraphical geologist. 1 

 But in addition to this work which it was at first called upon to per- 

 form, palaeontology has a more important role in the future history of 

 science, in demonstrating the steps in the evolution and succession of 

 faunas ; and on this basis its evidence must always be important in 

 forming a useful geological classification of strata. 2 It also contributes 

 important evidence of physical changes which took place in adjacent 

 regions. But the physical and palseontological evidences rarely coin- 

 cide ; so that for some time to come Stratigraphical classifications 

 should be made independently, first upon the evidences of the Physi- 

 cal History of a Region, and secondly upon its Succession of Life. 



1 Twenty-five years ago we began to give effect to a different view by arrang- 

 ing the collections in the Woodwardian Museum, in the University of Cambridge, 

 in the local faunas in which they were collected, since it was thus possible to trace 

 the migration of life step by step as it became diffused on the earth in conse- 

 quence of physical changes. This aspect of palaeontology was originally suggested 

 by the study of Count Munster's and other palseontological collections in the 

 Woodwardian Museum, and its importance has since been enforced by studies of 

 many collections in different parts of Europe. 



2 It is many years since Mr. R. A. C. Godwin-Austen wrote " The artificial 

 scale of formations which still figures in elementary treatises, more particularly 

 with respect to Secondary Geology, represents an order of superposition and lines 

 of separation which are both untrue as well with respect to the mineral masses as 

 the forms they contain, the result of too hasty generalisation of local phenomena ;" 

 but notwithstanding some improvements the criticism remains still just. 



