1912.] STEVENSON— THE FORMATION OF COAL BEDS. 521 



were obtained from the limestone at the Mercer horizon but they 

 abound in the calcareous shale ; the Brush Creek limestone and black 

 shale has abundance of brachiopods and pelecypods, the latter espe- 

 cially numerous in individuals ; the Ames limestone has few pelecy- 

 pods, specimens of the six recorded species being rare everywhere; 

 whereas the underlying shales have yielded 17 species, most of them 

 represented abundantly — and with them are several species of 

 brachiopods. Wholly similar conditions exist in Devonian and 

 earlier formations where pelecypods abound in sandstones and even 

 in calcareous muds but are comparatively rare in the limestones. 



Gastropods of the present day have great vertical as well as 

 geographical distribution, but many genera and species of other 

 genera have very limited vertical range. Respecting those of the 

 Pennsylvanian, little can be said. Their mode of occurrence gives 

 little clue to their habits, for many of the species are almost equally 

 abundant in shale and limestone. The migratory tendencies of 

 gastropods makes the evidence of scattered, even that of compara- 

 tively numerous individuals of doubtful value; but where a clay 

 shale is crowded with specimens of two or three species, as is the 

 case many times, there is little room for doubt respecting the habitat 

 preferred by those species. One who has made collections at several 

 localities would not hesitate to assert that many species of gastro- 

 pods found in the Allegheny and Conemaugh preferred to live near 

 the ingress of muddy water. 



If one were to conclude from the occurrence of cephalopod re- 

 mains, he might decide that the character of the water has been a 

 matter of indifference, for, from the earliest appearance of those 

 forms, they have been distributed in limestones, shales and sand- 

 stones, sometimes attaining great size in the last. In so far as the 

 Coal Measures of Pennsylvania and Ohio are concerned, the writer 

 has obtained few specimens from the limestones but he has collected 

 very many from the shales. The distribution would indicate that 

 muddy water was preferred. But this inference has no good basis. 

 Cephalopods are migratory ; modern types prefer pure water ; but 

 after death, the shell freed from the animal may float great distances 

 to be dropped in any kind of bed. The presence and the distribu- 

 tion of cephalopods give no certain information respecting the con- 



