Handbook of Paleontology 229 



in order to supply food for the abundant marine life of 

 the time. Algae or seaweeds secreting lime played an 

 important part in the formation of some of the Cam- 

 brian limestones. All the main stocks of invertebrate 

 life are represented in the Cambrian fauna which 

 ranges from simple sponges to highly developed crus- 

 taceans. The trilobites were the most striking forms 

 of life of the period but a number of other crustacean 

 groups, representatives of which live today, were found 

 in the Middle Cambrian (Burgess shale) of British 

 Columbia. The brachiopods came next to the trilo- 

 bites in abundance. There are two divisions of the 

 brachiopods: (1) with hingeless and horny (phosphate 

 of lime) shells ; (2) with calcareous shells and well- 

 developed hinges. Representatives of the first or more 

 primitive division are more abundant in the Cambrian ; 

 of the second division, in the later Paleozoic. Twenty- 

 two Lower Cambrian brachiopod genera have been de- 

 scribed for Europe and North America. Brachiopods 

 as a class are important throughout the Paleozoic and 

 make good index fossils because of the abundance of 

 individuals and because so many species have a short 

 vertical range, that is, are confined to one period or a 

 subdivision. No true pelecypod or mussel shells are 

 known in beds older than Ordovician (St Peter sand- 

 stone). Shells so classified hitherto are crustaceans. 

 Gastropods were represented by the cap-shaped or patel- 

 loid forms and also coiled forms. Real patelloids and 

 coiled Bellerophontids and the peculiar left-handed 

 (sinistral) coiled Scaevogyra came in for the first time 

 in the youngest of the fossiliferous Cambrian forma- 

 tions (Trempeleau of Mississippi valley). The typical 

 Scenellas of the Lower Cambrian have no unquestion- 



