I9I2.] STEVENSON— THE FORMATION OF COAL BEDS. 519 



their first appearance and, in all probability, as little in their habits. 

 P'or the most part, they cling closely to the strand in recent times. 

 Articulated brachiopods of the present day require deeper water and 

 84 of the living 129 species belong between 90 and 600 feet. Rhyn- 

 chonellids are never found in shallow water ; but this was not always 

 the condition, for in the older rocks they often occur abundantly 

 with thick-shelled lingulids in coarse sandstones and mud beds. The 

 terebratulids range from between tides downward. Of all living 

 brachiopods, 81 per cent, are bound to shallow waters, 7 per cent, 

 are found in the deeper waters of continental shelves and only 1 1 

 per cent, occur at greater depths. 



Applying Schuchert's results to the Appalachian basin, one finds 

 that Lingula and Orbiciiloidea (Discina of authors) are common in 

 the dark roof shale of the Middle Kittanning coal bed of Ohio and 

 that Lingula is found in several roof shales within Kentucky. It is 

 present at the Brush Creek horizon. Orhiculoidea is reported from 

 the Mercer and the Ames, one species being common to both, and 

 the same genus has been obtained from the upper part of the muds 

 and shales following the Ames. These forms are not numerous in 

 the limestones but they abound in some shales. The individuals are 

 seldom more than five eighths of an inch long but that size is excelled 

 only by forms in coarse rocks. Of the articulated brachiopods, the 

 rhynchonellids, which now prefer deep cold water, and the tere- 

 bratulids, most of which now inhabit the deeper shallow zone, are 

 feebly represented in the Pennsylvanian — perhaps because these 

 brachiopods were little differentiated at that time. Of the other 

 types, practically all belong to families now extinct and one must 

 determine their habits by the record which they have left in the 

 rocks. The productids attained noteworthy size in the later De- 

 vonian coarse sandstones, where they are associated often with 

 heavy-shelled Lingula or Discina. They are abundant in both the 

 pure and the muddy limestones of the Maxville. Eight species of 

 Productus are reported from the limestones and shales of the Penn- 

 sylvanian within the bituminous region, one being common to all 

 the lists, 2 to four of them, 2 to three, two belonging to the Cone- 

 maugh and one being confined to the Vanport. Five species are 

 found in limestone and shale alike and two are abundant in shale 



PROC. AMER. PHIL. SOC. LI. 207 G, PRINTED DEC. 16, I912. 



