48 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 76 



with the exception of Lonchodkmius and Reniopleimdes^ in the Lower 

 and Middle Chazyan and the Holston and Ottosee formations of the 

 Blount group in the southern Appalachian belts. Seperation of Pale- 

 ozoic faunas according to their geographic origin is discussed at con- 

 siderable length in following pages. 



Tt'ilohites pre fenced to graptolites in trans- Atlantic correlations. — 

 In my estimation Teleplius and the other above-mentioned genera of 

 Atlantic trilobites are at least as dependable criteria as the highly 

 esteemed graptolites in correlating formations in America with those 

 in Europe. Still these problems are never simple ; and too often the 

 fossil evidence is not as definitely indicative of time relations as it 

 may seem. In a recent paper ^^ I showed that the graptolites — mainly 

 because their preservation is usually too imperfect to permit of the 

 required intensive study and comparison of minute structural de- 

 tails — are as yet only a rather coarsely graduated standard of meas- 

 urement. Nor is the evidence of the trilobites or of any other class of 

 fossils easily evaluated. Closely similar species are found on the two 

 sides of the Atlantic, but other means must be employed before we 

 may be warranted in concluding that the observed slight differences 

 between the compared forms indicate merely locally developed con- 

 temporary modifications or that they are variations from type that 

 required long periods of time to produce. And even when two occur- 

 rences on opposite sides of the sea can not be satisfactorily distin- 

 guished the fact by itself is not determinative as to their practical 

 contemporaneity unless the remains are complicated in structure and 

 they agree in biblogically unimportant structural features.^'' It is 

 only after the finest possible differentiation of congeneric species or 

 varieties has been carried out that detailed correlation of their respec- 

 tive zones is validly permissible. 



In the Appalachian Valley, from central Alabama to, say 

 Staunton, Va., the occurrence of specifically identical forms of the 

 trilobite genera mentioned above may be accepted as reasonably 

 conclusive proof of the essential contemporaneity of the beds con- 

 taining them. Corroboration of the validity of this conclusion is 

 found in the fact that the species of Telephus and other genera 

 of trilobites that are regarded as belonging only to the zone of the 

 Whitesburg limestone are always found only beneath the lowest oc- 

 currence of the Normanskill graptolites and of the trilobites that in 

 southern Appalachian belts commonly occur either in the same layers 



18 Relative values of criteria used in drawing the Ordovlcian-Silurian boundary, Geol. 

 See. America Bull., vol. 37, p. 301, 1926. Because tbey tend to amplify facts presented 

 in this paper concerning diflferences on the two sides of the Atlantic in the vertical range 

 of graptolites that in Britain are regarded as reliable horizon markers it seems worth 

 while to direct the reader's attention to paragraphs on pp. 179 and 180 of Troedsson's 

 1928 work on the Middle and Upper Ordovician Faunas of Northern Greenland. 



»» See Ulrich, E. O., Correlation by displacements of the strand line, Geol. Soc. America 

 Bull., vol. 27, p. 488, 1916. 



