GLASS-SPONGE COLONIES OF THE DEVONIAN 35 



clothed in their new differentials. No shallowing of the sea or 

 positive diastrophy is required for this explanation. By the time 

 the Chemung outburst of species was effective all egress to or 

 access from deep water was shut off. Examination of Schuchert's 

 paleogeographic maps of this time will bring out this condition 

 clearly. There was no deep-water Devonian in the vicinity at that 

 period; to the east and south lay the Appalachian lands; to the 

 north Laurentia, and on the west a long and, we must say, putative 

 channel reaching in from the Pacific border. Through this channel 

 the sponges may have gone out. 



We conclude that the long evolution of these sponges from their 

 appearance in the dark Cambrian and Ordovician muds to their 

 immigration in the late Devonian was passed in the deeper waters 

 of the continental edge and is recorded in sediments beyond the 

 present reach of our observation. 



Barrois discovered the Dictyosponges in the Psammites du 

 Condroz of Jeumont in Brittany in sandy sediment at a horizon 

 equivalent to the Chemung of New York, and four of these species 

 in three genera were described and illustrated by Hall and Clarke 

 {op. cit). This is interesting collateral evidence of the widespread 

 influence which in the Northern Hemisphere impelled these sponges 

 on to the platform seas. 



WHY AND WHERE DID THE CHEMUNG DICTYOSPONGES GO? 



The course of their ontogenetic development shows that their 

 later expression assumed gerontic and adaptive characters in great 

 variety. The stratigraphic record indicates that the sandy bottom 

 on which the New York colonies and their contemporaneous species 

 grew was overwhelmed by incursions of coarse gravel washed in 

 from the rivers of the eastern Appalachian land. These terminated 

 their local existence and the Devonian period as well. Their emi- 

 gration from southern New York was westward and into deeper 

 waters of the Waverly group of western Pennsylvania and Ohio 

 and the Keokuk lime muds of Indiana. In these Mississippian 

 sediments they make their last appearance. But they were on 

 their way down to deeper waters and we find no reason against the 

 assumption that it was the westward course they followed on to 



