284 GEOLOGY. 



which are regarded as the highest class of mollusks have been found in 

 the uppermost beds referred to the Cambrian, but none as yet in the 

 lower part. As the cephalopods were even then highly developed 

 structurally, and were widely differentiated from the other mollusks, 

 there is little ground to doubt that they had already passed through a 

 long period of development, and hence that they were really present in 

 some part of the globe during the earlier Cambrian times. Their 

 remains are so rare that they have little geologic importance in this 

 period, but become prominent in the next. 



The pelecypods (bivalves) were represented in the Lower Cambrian 

 beds, though they have not been found abundantly. Whether this 

 implies that they did not live abundantly in the seas that made the 

 deposits that are now accessible, or that they were but poorly preserved, 

 is not known. Fig. 120, b, represents a typical form. 



Gastropods (univalves) (Fig. 120, c, d, and e) were somewhat abun- 

 dant even in the earlier Cambrian times, and became more common 



Fig.' 120. — Cambrian Mollusca. a, Ilyolithes americanus Billings, a pteropod of the 

 Lower Cambrian; b, Fordilla troyensis Barrande, a pelecypod of the Lower 

 Cambrian; c, Stenotheca rugosa, var. paupera Billings, d, S. rugosa Hall, capulid 

 gastropods of the Lower Cambrian; e, Trochus saratogensis Walcott, a Cambrian 

 gastropod with well-developed spire. 



toward the close of the period. The early forms found are chiefly of 

 the low conical type, known as capulids, from their resemblance to the 

 form of cap typified by the conventional " liberty-cap" (see Fig. 120, d, 

 in particular). The more amply coiled and spiral forms (Fig. 120, e), 

 though present, only became common later. The close resemblance of 

 Trochus saratogensis (Fig. 120, e) and other forms to modern gastropods 

 is worthy of note as showing how near to the present development 

 some of the Cambrian mollusks had attained. 



The Cambrian Molluscoidea. — This branch was well represented by 

 brachiopods (lamp-shells), (Fig. 121). In geological importance, these 

 rank second only to the trilobites in Cambrian times. Unlike the 



