268 SCHUCHERT— BLACK SHALE DEPOSITION. [May 7, 



It is apparent from the above that the common fossils are here 

 again those of the nekton (saurians, fishes with most of the ganoids 

 probably of freshwater habitat, belemnids, sepias) and drifted 

 land plants. Of the benthos, only a few species of bivalves are 

 common, and, while the ammonids are also bottom-dwellers and 

 occur commonly as fossils, their empty shells were probably drifted 

 into this black sea. The crinids were also drifted in, for the only 

 specimens found attached are on conifer wood, hanging head down- 

 ward; otherwise roots of these pentacrinids do not occur. 



In general it may be said that the Liassic deposits and the habitat 

 of the fossils of the time of Posidonomya hronni agree best with those 

 of the present Black Sea. Since this is true, it follows that the physical 

 conditions of the hronni sea must have been very much like those 

 of the Black Sea, i. e., it was a Liassic Black Sea into which drained 

 rivers, causing the surface waters to be more or less freshened, and 

 bringing land plants, logs, and ganoid fishes. However, there are 

 also marked differences, chief among which is the far less amount 

 of decomposition of the soft parts of ichthyosaurians and sepias, 

 of which fleshy parts are often preserved, a condition that never 

 occurs in the Kupferschiefer. Finally, the abundance of the Liassic 

 bivalves points to the shallow waters of the Modiola ooze of the 

 Black Sea, and therefore to depths of less than 600 feet. 



It seems to the reviewer that the present Black Sea, with its 

 great depth and widespread foul conditions, is an exceptional 

 example, and that in all of its features it may have no fossil ana- 

 logue. The Kupferschiefer and P. hronni seas along with the 

 American Ohio sea of Upper Devonian time and the Chattanooga 

 sea of the Mississippian period appear to agree with the essential 

 conditions of the Black Sea, except as to depth. All of the fossil 

 Black Seas appear not to have been deeper than 600 feet. 



Foul bottoms are clearly due to a lack of water circulation, either 

 because there is no wide connection with the oceanic areas or be- 

 cause there are inadequate vertical or convection currents. The 

 latter conditions may have been more abundantly attained in warm 

 climates than in cool ones, due to the fact that the heavier colder 

 waters sink to the bottoms and so oxygenate them. In this the 

 present is the exceptional condition when compared with most of 



