348 



GEOLOGY. 



their early climax less significant. The climax of the trilobites may 

 therefore be regarded as the foremost great event of its kind that is 

 definitely recorded. 



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Fig. 156. — The two upper curves represent the history of the trilobites according 

 to genera, the full line indicating the total number of genera present in the faunas 

 of the successive periods, and the dotted line the number of new genera introduced. 

 The two lower curves present the same data for the families of the trilobites, the 

 full line representing the total number of families present, and the dotted line 

 the number of new families introduced. The data for the families is taken from 

 Beecher in the Zittel-Eastman text-book of Paleontology, Vol. I. The data for 

 the genera is somewhat incomplete, but is as full as can be made from Zittel's 

 " Handbuch der Palseontologie." 



The general aspect of the trilobites at the high tide of their career 

 is fairly illustrated in the group shown in Fig. 157. The Ordovician 

 trilobites were usually much better preserved than their Cambrian 

 predecessors, but still they are never known to have been perfectly 

 fossilized. So general was the loss of the more fragile parts, that 

 the nature of the appendages and of the under parts was a mooted 

 question until recent years, when the under-structure was worked 

 out in the case of a few forms by the skill and patience of Walcott, 

 Beecher, and others. 1 These few forms doubtless represent the general 

 nature of the appendages of the whole order, though notable varia- 

 tions are not improbable. The accompanying photograph of a model 



1 Walcott, 28th Rept. N. Y. State Mus. Nat. Hist., 1875; Bull. Mus. Comp. Zool., 

 Cambridge, VIII, 1881. Beecher, Am. Jour. Sci., Vol. XLVI, 1893; Vol. XLVII, 

 1894, and Vol. I, Ser. 4, 1896; Am. Geol., Vol. XIII, 1894, and Vol. XV, 1895. Mat- 

 thew, Am. Jour. Sci., Vol. XLVI, 1893. (Weller). 



