540 HISTORY OF GEOLOGY. 



insular or continental, tropical or temperate. And M. Agas; iz, by the 

 examination of an incredible number of specimens and collections of 

 fossil fish, has been led to results which, expressed in terms of his own 

 ichthyological classification, form remarkable general laws. Thus, ac- 

 cording to him, 6 when we go below the lias, we lose all traces of two of 

 the four orders under which he comprehends all known kinds of fish ; 

 namely, the Cyclo'idean and the Cteno'idean ; while the other two orders, 

 the Ganohdean and Placo'idean, rare in our days, suddenly appear in 

 great numbers, together with large sauroid and carnivorous fishes. 

 Cuvier, in constructing his great work on ichthyology, transferred to 

 M. Agassiz the whole subject of fossil fishes, thus showing how highly 

 he esteemed his talents as a naturalist. And M. Agassiz has shown 

 himself worthy of his great predecessor in geological natural history, 

 not only by his acuteness and activity, but by the comprehensive 

 character of his zoological philosophy, and by the courage with which 

 he has addressed himself to the vast labors which lie before him. In his 

 Report on the Fossil Fish discovered in England, published in 1835, 

 he briefly sketches some of the large questions which his researches 

 have suggested ; and then adds, 7 " Such is the meagre outline of a 

 history of the highest interest, full of curious episodes, but most diffi- 

 cult to relate. To unfold the details which it contains will be the 

 business of my life." 



[2nd Ed.] [In proceeding downwards through the series of forma- 

 tions into which geologists have distributed the rocks of the earth, 

 one class of organic forms after another is found to disappear. In the 

 Tertiary Period we find all the classes of the present world : Mam- 

 mals, Birds, Reptiles, Fishes, Crustaceans, Mollusks, Zoophytes. In 

 the Secondary Period, from the Chalk down to the New Red Sand- 

 stone, Mammals are not found, with the minute exception of the mar- 

 supial amphitherium and plmscoloihermm in the Stonesfield slate. In 

 the Carboniferous and Devonian period we have no large Reptiles, 

 with, again, a minute amount of exception. In the lower part of the 

 Silurian rocks, Fishes vanish, and we have no animal forms but Mol- 

 lusks, Crustaceans and Zoophytes. 



The Carboniferous, Devonian and Silurian formations, thus con- 

 taining the oldest forms of life, have been termed paleozoic. The 

 boundaries of the life-bearing series have not yet been determined ; 

 but the series in which vertebrated animals do not appear has been 



Greenough, Address to Cfeol. Soc. 1835, p. 19. T Brit. Assoc. Report, p 72. 



