CAMBRIAN FAUNA. 141 



supremacy of their predecessors is made manifest. With the mid- 

 dle of the Devonian period the beginning of the trilobitic decay 

 becomes apparent, and, after the close of that period, i. e., in the 

 Carboniferous, less than a half-dozen types remain, and even these 

 are of comparatively rare occurrence. At the close of this last- 

 named period the trilobites disappear totally and forever from the 

 scene.* 



Broadly looking over the Cambrian fauna, \ve find it to be dis- 

 tinguished by two important features. One of these is the fact 

 that it is entirely destitute of both land and fresh-water forms, or 

 such as are strictly adapted to breathing directly the oxygen of the 

 atmosphere or that of fresh water. All the forms thus far encoun- 

 tered are, as far as we know, of a strictly marine nature. The ab- 

 sence of land animals will scarcely appear surprising in view of the 

 complete, or nearly complete, absence of a land vegetation, and the 

 correlative want of the nourishing material requisite for that charac- 

 ter of organisms. The absence of fresh-water forms is not so readily 

 accounted for, unless it be that there were formed at that time no 

 fluviatile or lacustrine accumulations of sufficient magnitude to have 

 left their traces behind them. It is not impossible, however, that 

 some, or even many, of the recognised marine fossils, or such as 

 have a marine habit, of the Cambrian formation, are in reality 

 estuarine or brackish forms, as it can scarcely be conceived that all 

 the deposits that were formed at the mouths of the ancient rivers 

 should have been so totally destroyed or covered over as to have 

 left absolutely no vestiges of their former existence. Doubtless, 

 some of these have been preserved, along with their contained fos- 

 sils, although the exact nature of such deposits may be disguised 

 from us by reason of our imperfect knowledge concerning the true 

 habits of their representative organisms. Nor would it be abso- 

 lutely safe to affirm that some of these organisms, undistinguishable 

 from what at the present day are indisputably marine types, may 

 not in reality have been of a purely fresh-water habit in those early 

 days. 



The other distinguishing feature of the Cambrian fauna is the 



* Shumard has described representatives of the genera Phillipsia and 

 Proetus in deposits of the Sierra Madre, of the Southern United States, claimed 

 to belong to the Permian period ; the determination of age may be considered 

 to be very doubtful, however. 



