360 CHARLES R. KEYES 



and the Cottonwood limestone of the central part of the last 

 mentioned state. 



A careful comparison of the species of fossils that are con- 

 sidered by Smith from the "Upper Coal Measures " with those 

 from the Upper and Lower Coal Measures of other parts of the 

 Western Interior basin brings out the fact that the Arkansas 

 fauna not only does not necessarily indicate a "very high" 

 position in the Upper Coal Measures of the zone containing it, 

 but that it may be, and probably is, very low. Judging from the 

 fauna alone, and as a whole, according to the palaeontological 

 standard of its nearest and most closely related districts — the 

 Missouri-Kansas province, with which it is properly compared — 

 the indications are that the age of the strata yielding it is not 

 that of the "Upper Coal Measures" at all, but of the "lower 

 division" — that is, of the Des Moines series of the more north- 

 ern localities. 



All of the Arkansas species, with very few exceptions, are, in 

 Iowa, Missouri and Kansas, the most widely distributed forms. 

 Most of them range from the base to the top of the Des Moines 

 series, and continue on upwards. In the lower series the marine 

 beds are almost wholly absent, only a few thin limestones being 

 present in the whole succession. Nevertheless the same species 

 that are found in Arkansas occur abundantly not only in the 

 thin limestone layers, which are rarely more than a few inches 

 in thickness, but also in the calcareous shales, and, in less num- 

 bers even in the bituminous shales. 



Of the corals listed from Arkansas only one form has a 

 range that is unusually "high" in the northern succession; all 

 of the others start almost from the very base of the series. The 

 crinoids and bryozoans are all common in the Lower Coal Meas- 

 ures. All fourteen species of brachiopods are of very frequent 

 occurence in the lower coal division, many commencing down 

 in the Mississippian. One possible exception is Terebratula 

 bovidens, which at present appears to be absent from some of 

 the lower Des Moines beds. Of the lamellibranchs, all twenty- 

 two species are the most characteristic forms of the very base 



