LIFE 555 



The old types of fishes gave place to new ones (the teleosts) dur- 

 ing this period. This change set in during the Comanchean, and 

 was complete by the middle of the Cretaceous, though representa- 

 tives of the older types lived on. 



Invertebrates. The most notable departure from the preceding 

 is the prominence of foraminifers among the fossils. They 

 made large contributions to the chalk of the period, and they were 

 concerned in the formation of the greensand, scarcely less char- 

 acteristic of the period than the chalk. While some of these mi- 

 nute organisms live on shallow bottoms, on fixed algae, and in 

 abysmal water, they are chiefly inhabitants of the surface waters of 

 the open sea. 



Sea-urchins (a-e, Fig. 469) were quite abundant, and lent one 

 of its characteristic aspects to the fauna. Corals and crinoids, so 

 long associated with clear seas, were not plentiful. In the clastic 

 formations, pelecypods (/-A) and gastropods (i-l) abound (Fig. 469). 

 It \\ill be seen by a glance at Fig. 469 that they have a modern 

 appearance. Cephalopods were still abundant, though ammonites 

 \\ ( i r in their decline and showed erratic forms, attended by excessive 

 ornamentation, comparable to that which marked corresponding 

 stages of the trilobites and crinoids. Odd forms of partial uncoiling, 

 or of spiral and other unusual forms of coiling, were common (Fig. 

 470). Interesting forms, perhaps to be classed here, were the 

 Kaciditcs (i), which resumed the straight form of the primitive 

 Ort/ioceras, while retaining the very complicated sutures of the 

 Ammonites (c). 



Map work. Folios of the U. S. Geol. Surv. and Laboratory Exercises in 

 Structural and Historical Geology, Exercise XI. In the folios of the U. S. Geol. 

 Surv. both Comanchean and Cretaceous are classed as Cretaceous, though the two 

 are distinguished, in some cases, in the text and on the maps. 



