QQ PROCEEDINGS OF THE NATIONAL MUSEUM vol.76 



arisen mainly from the fact that in the Drummuck and Keisley of 

 Britain, the Leptaena limestone of Sweden, and to lesser extent in the 

 Borkholm of Estonia the, as I think, really Silurian species of the 

 above-listed genera of brachiopods and trilobites are associated with 

 a predominating number of direct descendants of preceding stages of 

 the middle Atlantic fauna found in underlying really Ordovician 

 formations in the same countries. Because of the dominance of these 

 persisting, "residual" Ordovician elements— among them even a 

 species of Telephus — it is only to be expected that in following the 

 formerly prevailing but now thoroughly discredited method of deter- 

 mining the age of a formation by the relative dominance of generic 

 and specific similarities in compared faunas geologists generally as- 

 signed these legitimately debatable formations to the Ordovician. 

 However, in doing so they caused much regrettable confusion in 

 stratigraphic correlation and great but I hope only temporary impair- 

 ment of the indexical value of more than 30 genera of fossils. In 

 making this statement I do not wish to be understood as implying 

 that the vertical range of these generic types is fixed and chronologi- 

 cally the same the world over and that they did not exist somewhere 

 in recognizable form either earlier or to later dates than the informa- 

 tion now available indicates. On the contrary, I feel certain that the 

 30 genera just referred to originated and slowly developed their char- 

 acteristics in southern marine realms, the Silurian life of which we 

 know now only through the little we can gather from the migrants 

 that reached and left their remains in epicontinental basins of the 

 northern hemisphere. 



SiluHan age of the Leptaena limestone proved hy graptolites. — 

 Facts that give me much satisfaction because they will be accepted 

 generally by European geologists as proving the Silurian age of the 

 Upper Leptaena (Kallholn) limestone and its equivalents or near 

 equivalents elsewhere, and also of the underlying Dalmanites shale 

 have recently been brought out by Troedsson and Roswall who found 

 that black shale with Middle Birkhill graptolites are really inter- 

 bedded with the reefy deposits of the former at Kallholn, Dalarna, 

 Sweden. So much of the battle seems thus to have been won. But 

 why not go a step or two further down in the section and drop the 

 Ordovician-Silurian boundary to some still lower diastrophically 

 marked plane that would more nearly correspond to the naturally 

 defined and very widely recognizable stratigraphic break in the 

 American section? For instance, to the base of the Staurocephalus 

 zone ; or even to the bottom of the " Trinucleus beds ? " The latter, 

 like the Middle Richmond Sylvan shale in Oklahoma, contains a 

 species of Dieellograptus that is referred to the British Upper Hart- 

 fell D. comploxnatus ; and this is not by any means the only fossil 



