GLASS-SPONGE COLONIES OF THE DEVONIAN 29 



Ground habitat. — The nature of the ground determined for loi 

 of these species is given by Schulze as follows : 



Material Number of Species 



Sand 5 



Gravel and stones 2 



Hard ground 6 



Coral mud 7 



Volcanic mud 14 



Green mud i 



Red mud 2 



Mud (including blue mud) 32 



Red clay 11 



Globigerina ooze 13 



Pteropod ooze 7 



Radiolarian ooze 2 



Diatom ooze 9 



Schulze observes that forms equipped with root tufts were 

 principally found in soft muddy ground, and in the Devonian seas 

 the species generally were provided with a more or less conspicuous 

 tuft of this kind. 



Temperature habitat. — The temperature of the Chemung period 

 was probably pretty cool. Glacial ice had formed over the ele- 

 vated land of the middle Devonian on the Atlantic border of this 

 continent and now with partial resubmergence the refrigerated 

 waters were discharging themselves, with abundant landwash, 

 into the shallow seas. In the Portage division of the late Devonian 

 an immigrating warm water fauna (the Manticoceras intumescens 

 fauna) coming in from the west was blocked and stopped on its 

 way, driven out or destroyed by the presence of the cool waters 

 carrying the Chemung fauna. 



Today the hexactinellids show an apparently different tempera- 

 ture control, as witness the "Challenger's" record: 



North Temperate zone 20 species 



Tropics 45 species 



South Temperate zone 35 species 



This apparent difference is compensated by the cold of the deep 

 waters. 



