GEOLOGY OF THE LOWER AMAZON REGION 729 



type, Leptaena rhomboidalis, Schuchertella missouriensis (earliest ap- 

 pearance of this genus), Rhynchotrema n. sp. (near inaequivalvis. No 

 R. capax occurs in this fauna, the diagnostic species of the Richmond 

 formation), R. near dentata^ Triplecia n. sp. (a form externally with 

 the characters of A try pa rugosa or A. marginalis), Clorinda (all other 

 forms of this genus are Siluric), A try pa near marginalis (a decided 

 Siluric reminder), Zygospira putilla, Cypricardinia near arrata 

 (earliest occurrence of this genus), Conradella near dyeri, Encrinurus 

 of Siluric character, Calymene near niagarensis, and Homalonotus. 

 In other places at about this same horizon have been found Halysites, 

 Lingulops, Pholidops, Orthis near callactis and Rhynchotreta near 

 cuneata. This fauna therefore immediately suggests the BraziKan 

 Siluric and hnks it unmistakably also with the Richmond below, 

 but less so with the Clinton above. More recently Ulrich has re-exam- 

 ined the Medina deposits of the Appalachian region, more especially 

 in Pennsylvania, Virginia, and Tennessee, and has concluded that 

 they are the eastern shore deposits equivalent to the Richmond series 

 of the Ohio and Mississippi valleys. This result therefore forces 

 stratigraphers to place the line separating the Siluric from the Ordo- 

 vicic, not at the base of the Medina formation of the New York 

 standard section, but at its uppermost Kmit and beneath the Clinton. 

 Arthrophycus harlani, which should be written A. alleghaniensis 

 (Harlan), and Daedalus archimedes'^ are therefore not the guide fossils 

 to indicate the base of the American Siluric, but mark the marginal 

 littoral facies of the sea toward the close of the Ordovicic. This refer- 

 ence of Arthrophycus to the Ordovicic is in harmony with its similar 

 occurrence in Portugal. It will eventually be shown by Ulrich that 

 the Medina is Ordovicic in age, and when his work appears it will 

 be seen that the Brazilian fauna fits in well with the Mississippi 

 valley highest Richmond, and that the lost time interval between the 

 Ordovicic and Siluric is not long. This reference of these formations 

 to the Ordovicic is also in harmony with the sequence in Russia along 

 the shore of the Baltic Sea. Here the Lyckholm and Borkholm beds 

 have many Siluric corals and brachiopods, but the European stratig- 

 raphers always regard these formations as belonging to the Ordo- 



I Sarle, "Arthrophycus and Daedalus of Burrow Origin," Proceedings of the 

 Rochester Academy 0} Sciences, Vol. IV (1906), pp. 204-10. 



