364 



HISTORICAL GEOLOGY 



FIG. 374. Two common 

 Devonian pelecypods. 



adverse or even fatal to another. Thus many of the mollusks 

 prefer somewhat turbid water and a muddy bottom, while 

 the corals are exterminated by any 

 large admixture of sediment in the 

 water hi which they live. With this 

 principle in mind we shall be prepared 

 to find the crinoids, corals, and other 

 animals which were abundant in the 

 Niagara sea relatively uncommon in 

 the Devonian formations except the 

 limestones. 



Mollusks and brachiopods numerous. 

 Their places were taken by hosts of 

 two-shelled mollusks (Fig. 374) and 

 brachiopods (Figs. 375 and 376), with 

 other groups in subordinate positions. 

 The brachiopods in particular were prob- 

 ably near their zenith in the Devonian. 

 Most of the important types had made 

 their appearance in full force, and it re- 

 mained for later periods only to carry out 

 the lines of progress already defined. 



Decline of the trilobi es. The trilobites 

 had by this time dwindled to a few forms 

 (Fig. 377) which, however, clung to their 



Silurian propensity for useless excres- 

 cences and ornaments. Although the 

 two cases may not be similar, there is 

 a resemblance to certain decadent 

 families among our own race who cling 

 to the traditions and outward appear- 

 FIG. 376. A large brachio- ances of former rank, long after they 

 pod common in the Devo- have been shorn of power and wealth. 



nian rocks (Spirifer). _ 



Cephalopods take a new line of ad- 

 vance. The chambered mollusks, or cephalopods, now enter 

 upon a new career which eventually leads them to the extreme 



Fig. 375. One of 

 the commonest De- 

 vonian brachiopods 

 (Atrypa). 



