252 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



(d) Bottom, clean or relatively free from deposits of silt. 



(e) Composition of the oceanic salts, probably the same as in the 

 ocean of the present day. 



(/) Specific gravity of the oceanic waters, probably about as in the 

 ocean of today. Certainly the specific gravity was high enough to fur- 

 nish the large quantities of calcium salts demanded by the reef organisms 

 for the formation of their skeletons. 



BEARING OF THE PALEOZOIC BRTOZOA ON PALEOGEOGRAPHY^ 

 BY E. 0. ULRICH 



The Bryozoa undoubtedly lead all other Paleozoic invertebrates except 

 the Ostracoda in abundance of individuals, and probably also in specific 

 differentiation. They occur more or less profusely in all kinds of de- 

 posits save in the coarser quartzose sandstones and in black shales. They 

 seem to have preferred waters depositing slightly argillaceous limestone. 

 Like their recent representatives, they flourished best in relatively quiet 

 waters and at depths little beyond the zone of violent wave action. In 

 their mature development and habits of growth they are essentially bot- 

 tom-dwelling sessile organisms, their calcareous colonies being attached 

 to stones, dead shells, and other foreign objects. It is for this reason 

 that they are absent or rarely found in black shales, in which littoral, and 

 in fact bottom-dwelling invertebrates of all sorts, are similarly infrequent. 

 Although usually fixed in their mature stages of growth, their larval forms 

 are free-swimming, and this fact, doubtless, accounts for the great geo- 

 graphic distribution often attained by species of this class. It also sug- 

 gests that their dispersal, which was greatly facilitated by currents, took 

 place rapidly, and in this lies their great value as horizon markers. In 

 fact no other group of organisms has proved of greater value in strati- 

 graphic correlation. 



In general aspect bryozoan colonies vary exceedingly. Some form 

 masses several feet in diameter and grow so profusely as to almost rival 

 the corals in reef-building. Some are hemispheric, others twiglike or 

 bushy, but more of them, especially of the middle and late Paleozoic 

 species, form very delicate lacelike fronds or incrustations. 



The Bryozoa seem to have originated in the Caribbean Sea or Gulf of 

 Mexico, the oldest representative being a species of Nicholsonella, found 

 in Canadian rocks in northern Arkansas, laid down by waters invading 



1 Manuscript received by the Secretary of the Society May 23, 1911. 



