28 JOHN M. CLARKE 



belonging to the advance guard of the Chemung army, as in various 

 respects the faunal relations of the two are close. 



HABITAT OF THE DEVONIAN SPONGES 



These glass sponges obviously grew on sandy bottom at a depth 

 which could not well have been more than one hundred fathoms, and 

 probably not more than fifty. The waters of the epicontinental seas 

 were always shallow; even the clay and lime muds betoken no depth 

 comparable to the deep-sea oozes and blue muds of the present 

 oceans. There is no single exception to this interpretation that 

 could carry any weight in the presence of such overwhelming proof 

 of the adaptation of this great array to conditions of life wholly 

 unlike those under which their successors are living. When the 

 Devonian time was over the simpler and typical expressions of 

 the obconical, prismatic, and nodose sponges disappeared and were 

 replaced by accelerated species of like stock (twenty species are 

 recorded by Hall and Clarke) in the Mississippian stage, the 

 Waverly and Keokuk divisions, in which there is a notable increase 

 of lime sedimentation and consequent evidence of a deepening sea. 



HABITAT OF LIVING HEXACTINELLIDS 



Depth habitat. — Generally and specifically, these are deep-sea 

 animals. Agassiz dredged them from 2,410 fathoms, but the 

 "Challenger" expedition, as shown in Schulze's report, determined 

 a greater range and a greater depth. The ''Challenger" garnered 

 about one hundred species, none of which grew at less than ninety- 

 five fathoms. The summary of the record is as follows: 



From 95 to 200 fathoms 24 species 



200 to 300 fathoms none 



301 to 700 fathoms 35 species 



701 to 900 fathoms none 



901 to 1,000 fathoms 2 species 



1,001 to 2,900 fathoms 47 species 



Thus all the species are of the deeps and many of the very great 

 depths. 



