THE CAMBRIAN PERIOD. 



281 



The trilobites were easily the most distinguished of the known deni- 

 zens of the Cambrian seas. They were not only the highest in organi- 

 zation, but the most characteristic of the age. Their successive genera 

 best distinguish the successive stages of the period, and their distri- 

 bution is a chief means of correlating the formations on different con- 

 tinents and in different provinces of the same continent, as previously 



Fig. 118. — Cambrian Trilobites. a, Holmia (Olenellus) brdggeri Walcott, a charac- 

 teristic trilobite of the Lower Cambrian; b, Crepicephalus texanus (Shumard), 

 a trilobite of the Middle Cambrian; c, Ptycoparia antiqua (Salter), a trilobite of 

 the Middle Cambrian. The above trilobites belong to the order Opisthoparia. 

 d, Agnostus obtusilobus Matthew, a Middle Cambrian trilobite of the lowest order, 

 Hypoparia, showing primitive characters, as the absence of eyes, small number 

 of thoracic segments, etc. 



set forth (p. 240). They have hence a leading place in historical and 

 geological importance. They belonged to a race long since extinct. 

 A study of the accompanying illustrations (Fig. 118) will best impress 

 their distinguishing features, notably their three longitudinal lobes, 

 whence their name, and their three transverse divisions, head, thorax, 



