74 



HISTORICAL GEOLOGY 



a Fig. 36 b 



Cambrian Brachiopods : a, Lingulella prima, 

 b, Lingulella acuminata. (After Walcott.) 



Trilobites (Crustaceans), are the most important Cambrian fossils 

 (Fig. 36). There are two important general groups of Brachio- 

 pods, namely, the Inar- 

 ticulates, in which the 

 horny shells or valves are 

 not joined together by a 

 hinge, and the Articu- 

 lates, in which the heav- 

 ier calcareous shells are 

 joined together by a hinge 

 structure. The former are 

 simpler and lower in or- 

 ganization, and, from the 

 standpoint of evolution, 

 it is important to note 

 that the Cambrian (and also pre-Cambrian) Brachiopods were 

 mostly Inarticulates, the Articulates not becoming common till 

 in the Upper Cambrian. 

 In the post-Cambrian 

 periods the Articulates 

 greatly outnumbered the 

 Inarticulates, and they 

 are the most common of 

 all fossil shells in the 

 Paleozoic rocks. The 

 Brachiopods stand out 

 conspicuously as a re- 

 markably persistent class of animals ranging from pre-Cambrian 

 time to the present, and, although there have been very many 



species and genera changes, 

 the class as such has been very 

 little changed. Many hun- 

 dreds of species are known 

 from the Paleozoic rocks alone, 

 and by studying their gradual 

 changes in species and genera, 

 they have come to rank among 

 the most valuable fossils as 

 geologic time markers and for 

 purposes of correlation. 



Fig. 37 

 A Cambrian Pelecypod {Fordilla troyensis). 

 Shell on right and cast on left, much en- 

 larged. (After Walcott.) 



,. 





a Fig. 38 b 



Cambrian Gastropods: a, MathereUa 

 saratogensis; b, Pelagiclla minutis- 

 sima. (After Walcott.) 



