358 



GEOLOGY. 



the asteroids, the ophiurians, and the echinoids had made their appear- 

 ance, though scantily represented. This leaves unrepresented but 

 one of the six classes susceptible of ready fossilization, the Mastoids, 

 and even these were foreshadowed by the blastoidean characters of 

 some of the crinoids. Of all these, the cystoids were the most primi- 



FiG. 164. — Ordovician Bryozoans. a, Constellaria polystomella Whitfield; b, Crept- 

 pora hemispherica Ulrich; c-d, Stomatopora delicatula (James); e-f, Rhinidictya 

 mutabilis (Ulrich); g-h, Monticulipora arborea Ulrich; £-/, Callopora pulchella 

 Ulrich; k, Phylloporina granistriata Ulrich. These figures represent a few of 

 the almost innumerable forms of bryozoans present in the Ordovician faunas. 

 At times they contributed a large portion of ^the calcareous matter of the 

 limestones and calcareous shales. 



tive in character and were doubtless most nearly like the parent stem 

 from which all were derived, if indeed the cystoids were not the ances- 

 tral type. The irregular, unsymmetrical forms of the cystoids (Fig. 165, 

 a-f) were gradually replaced by the more symmetrical and better 



