498 



GEOLOGY 



of the period were second to trilobites only; but unlike the trilo- 

 bites, brachiopods still live, and are conspicuous representatives of 

 stability and persistence. Though the species and most of the 

 genera have changed, the class as a whole has been but slightly 

 modified since the Cambrian period. Then, as now, the valves of 

 one division of the group were hinged, while those of another were 

 not. One division formed shells of calcium phosphate, and another 

 of calcium carbonate. 



Mollusca. Cephalopods (chambered shells), the highest class of 

 mollusks, are found in the uppermost beds of the Cambrian. As 



Fig. 374. Cambrian Mollusks: a, Hyolithes americanus Billings, a Lower 

 Cambrian pteropod; b, Fordilla troyensis Barrande, a Lower Cambrian 

 pelecypod; c, Stenotheca rugosa Hall, a capulid gastropod of the Lovyer 

 Cambrian; d, Trocus saratogensis Walcott, a Cambrian gastropod with 

 well-developed spire; e, Platyceras primcBvum Billings, a Lower Cambrian 

 gastropod; /, Ophileta primordalis Winchell, an Upper Cambrian gastro- 

 pod. 



they were even then highly developed, there is little doubt that the 

 class had passed through a long history before the latter part of 

 the period. 1 Pelecypods (bivalves, 6, Fig. 374) lived even in the 

 early part of the period, though their remains are not abundant. 

 Gastropods (univalves, c, d, e, Fig. 374) were rather plentiful 

 throughout the period. The early forms are chiefly of the low coni- 

 cal type,, while more amply coiled and spiral forms became common 

 later. The close resemblance of some of them to modern gastro- 

 pods is worthy of note. 



'Ulrich would refer the beds containing the Cephalopods to the 

 Ozarkian, a system which he would make to include the upper part of the 

 Cambrian and the lower part of the Ordovician, as usually classified. 



