Williams — Age of Manganese heds of Arkansas. 331 



Strophomena {Leptmna) Dalmanites limidurus. 



rhomboidalis. Encrinurus phlyetainoides, 



Meristina nitida. Illwnus ioxus. 



Comidites proprius. Lichas breviceps. 



Calymene niagarensis. Streptis grayi and others. 



The linking of the manganese deposit with this fauna is by 

 the species JLeptcena [Plectambonites) transversalis of the va- 

 riety elegantida, figured by Foerste (see reference above) 

 and presenting also the characters previously figured and de- 

 fined by Davidson, after Angelin as Leptmna segmentum Ang. 

 (Brit. Foss. Brae. vol. iii, PL xlviii, figs. 28-30.) This spe- 

 cies is from the Wenlock and Dudley. But when we look at 

 the specific peculiarities we find the same characters in a 

 slightly modified form in the Leptcena quinquecostata McCoy, 

 of the Bala and Caradoc. (See same plate, figs 23-27.) 



The association of species in either of our faunas is suffi- 

 ciently distinct to indicate a change of faunas during the in- 

 terval separating them, but when we examine each species 

 and note its already recorded range we are struck with the 

 number of species which have been recorded from both lower 

 Silurian (Ordovician) and upper Silurian rocks. The lower 

 Llandovery fauna of Europe, and the Clinton with us, have 

 referred to thern species which were first seen in the lower 

 rocks. And the Anticosti limestone is another case, present- 

 ing a mixture of the species generally found either at lower or 

 at higher horizons. The study of the fossils indicates there- 

 fore that the manganese ore is (a) associated with deposits of a 

 particular age, (b) at an horizon which is definite in the sec- 

 tions examined, (c) between two limestones containing two 

 distinct faunas, {d) the lower one of which contains Ordovician 

 species of which almost every species has been reported in rare 

 cases, here and there, in deposits which have been called upper 

 Silurian in age. (e) On the other hand, the fauna of the upper 

 limestone, while it contains an unmistakable upper Silurian 

 fauna, contains a few species which have been reported from 

 lower Silurian horizons. 



The locating of the age of the deposit of manganese near 

 to that represented by the Clinton of New York points to the 

 wide spread influence of the disturbance which closed the 

 sedimentation of the Ordovician (lower Silurian) for North 

 America. The elevation which left the Cincinnati axis above 

 sea level, affected the faunas of the Arkansas region less be- 

 cause further removed from large masses of land, and the 

 faunas above and below the interval differ less, as may be sup- 

 posed, because the ocean was near at hand, not more than a few 

 hundred miles, when the elevation was at its extreme. 



