346 P. E. Raymond — History of Corals, etc. 



habit. Being accustomed to cool water tliey survived the 

 Permian giaciation^ but due to geographic and climatic 

 changes, were perhaps driven from their original homes, 

 and, the temperature of ocean waters being universally 

 lowered, attained a wide distribution. They found the 

 geographical and ecological positions of the Tetracoralla 

 uninhabited, and so began to thrive and to change. 



Writers have repeatedly commented on the fact that 

 corals are exceedingly rare in the older Triassic deposits, 

 and reefs of Hexacoralla first appear after the middle of 

 Triassic time. It may be inferred from this that the 

 oceanic temperatures did not become favorable for corals 

 for some time after the melting of the Permian ice, and 

 by this time the limeless corals had become established 

 in many regions previously unoccupied by them. With 

 the warming of the waters there came a surplus of cal- 

 cium, and the animals no longer being able to fully elimi- 

 nate it, began to build skeletons. By this time the larval 

 form had been sufficiently changed so that septa were 

 deposited in cycles of six instead of four, and a new era 

 in the history of corals was inaugurated. 



It does not appear that the question of temperature, 

 and inferentially of climate, has been given sufficient 

 attention in connection with the discussion of the origin 

 of the ^' lime habit" in animals. The first great develop- 

 ment of calcareous skeletons was in the Ordovician, and 

 was coincident mth the first appearance of great masses 

 of non-clastic limestone, and with the first relatively com- 

 plete world-wide peneplanation. The almost universal 

 Pre-Cambrian mountain building indicates that the Cam- 

 brian sedimentation began with differentiated but proba- 

 bly cool climates, and while small deposits of limestone 

 formed during the Lower Cambrian testify to local, warm 

 basins, the universal rise in temperature came later. 



A second factor connected with the secretion of cal- 

 careous skeletons is that of the habits of the animal, 

 whether active or sedentary. Although there are excep- 

 tions to the rule, active animals usually have less thick 

 shells or bones, not because they have less need for them, 

 but probably because they eliminate lime more readily. If 

 one surveys the Cambrian fauna, he finds there few seden- 

 tary animals. Those which are present are sponges, a 

 few cystids, some protrematous and a few atrematous 



