■ 36 JOHN M. CLARKE 



the epicontinental margin. As Dictyosponges they have never 

 reappeared, nor as Lyssacine hexactinellids. That role was played. 

 When their successors came back in the Jurassic and Cretaceous 

 times their independent spicules had been fused into continuous 

 networks and, as Dictyonine sponges, they carried on their impor- 

 tant work as reef and rock builders. Seldom, however, did they 

 reproduce the form of their Devonian predecessors; indeed the 

 ancient form is far better revived in the glass sponges of today. 

 That, too, is an interesting illustration of once more passing the 

 same point on the cycle of their development history. 



SUMMARY 



1. In the Cambrian and Ordovician times the Lyssacine 

 hexactinellids grew freely in the black muds of moderately deep 

 epicontinental waters. 



2. They were on their way off the American continental plat- 

 form and down to the marginal seas. 



3. Here they carried out their evolution during the long Silurian 

 (when a stray species came ashore) and all the early stages of the 

 Devonian. 



4. In the later Devonian they return in great force and with 

 their evolution fully under way, but not to such an extent as to 

 conceal their ontogenetic stages and the radicle expression on which 

 they were based. To this reappearance on the epicontinent they 

 were evidently impelled by some vis a tergo, some compelling 

 external force, probably the invasion of their province by a dominat- 

 ing life-element of another kind. They were caught in a general 

 migration of the time into the shallow and cool waters of the 

 Chemung, and in these waters they perfected their evolution. 



5. Out of these shallow waters they were driven by an incursion 

 of fresh waters which flooded the Devonian province with gravel 

 from the eastern lands. 



6. They migrated thence westward into the deeper waters of the 

 Mississippian and from there once more to the circumcontinental 



seas. 



7. In their return to the epicontinents of the Jurassic and 



Cretaceous their development had advanced to a change of skeletal 

 structure and a wide variation of form. 



