THE CAMBRIAN PERIOD 



413 



nearly every particular they were very primitive or simple in struc- 

 ture and closely agree with a theoretical crustacean ancestor. 



Since trilobites moulted their shells at certain times and the great 

 majority of their fossils consist of these fragments, a complete speci- 

 men usually indicates the death of an individual. 



Not only were trilobites the most conspicuous animals of the period, 

 but since new species and genera appeared, while the older became ex- 

 tinct, they furnish the best means of correlating the formations of 

 different continents and of widely separated portions of the same con- 

 tinent. The three divisions of the system, as has been seen (p. 402), 

 are consequently named for the three dominant genera of trilobites : 

 the Lower or Olenellus zone, the Middle or Paradoxides zone, and the 

 Upper or Dicellocephalus zone. 



Other Crustaceans. — In addition to trilobites a number of crus- 

 taceans (Fig. 382 E, F) of a different group, representatives of 

 which are living to-day, have been found in the Middle Cambrian 

 of British Columbia. That a large and varied crustacean fauna pre- . 

 ceded these is certain. 



Mollusca 



Gastropods (Univalves). — This class is now represented by snails, 

 conchs, and winkles. The most conspicuous feature of the shelled 

 forms is the single, 

 usually spiral shell. 

 Gastropods lived 

 throughout the period 

 but were seldom abun- 

 dant. The earliest 

 forms were chiefly 

 simple, conical shells 

 (Fig. 383 C, E), while 

 later in the period 

 coiled and spiral forms 

 (Fig. 383 B, D) be- 

 came more common. 

 Some of the spiral 

 forms bear a close resemblance to some modern gastropods. 



A division of the gastropods, the pteropods, was well represented in 

 the Cambrian. The fossils usually consisted of simple, conical shells 

 (Fig. 383 A). Several specimens have been discovered with distinct 

 impressions of the characteristic fleshy portions. 





Fig. 383. — Cambrian gastropods: A, Hyolithes cari- 

 natus; B, Pelagiella {Platyceras) -primcevum; C, Scenella 

 varians ; D, Trochus saratogensis ; E, Stenotheca rugosa. 



