514 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



detailed correlation. The recurrences of a species or fauna may all be 

 limited to a single province but occasionally they extend into areas usually 

 distinct. All fossil species found in two or more successive beds are in 

 fact recurrent, but technically this term is applied only to species whose 

 periodic appearances are separated by considerable intervals in which they 

 are absent. In most cases the several occurrences are distinguishable, 

 and when properly discriminated are of high value in correlation. Good 

 examples are the varieties of Plectamhpnites sericeus, Rafinesquina al- 

 ternata, Leptaena rhomb oidalis, Platystrophia lynx, Cydonem,a hilix, and 

 Calymene callicephalus. Even when they are not distinguishable the 

 different occurrences may be of excellent stratigraphic service. This is 

 true especially of prolific species whose occurrences are separated by well 

 marked faunas. Tropidoleptus carinatus is a notable example. Another 

 that has proved very useful is the Orthorhyncula linneyi. The latter is 

 found very abundantly at two Ordovician horizons in Kentucky and mid- 

 dle Tennessee, the first being in the Catheys formation, the second near 

 the top of the Fairview formation. In its second invasion the species 

 extended northward along the Alleghany front to central Pennsylvania. 

 As the species is easily recognized and its second appearance limited to a 

 thin zone it is justly counted among the most reliable and useful of 

 guide fossils. 



(9) Time range of fossil species and genera. — The time range of fossils 

 is ever open to revision, hence no genus or species of fossils is to he 

 accepted as permanently referable to only a single unit of the strati- 

 graphic scale. A given fossil is diagnostic of a large or smaller unit, as 

 the case may be, only so long as it has not been found in either younger 

 or older positions. This principle is based on the obvious fact that lists 

 of characteristic fossil genera or species of any period, epoch or age are 

 merely expressions of the knowledge at hand concerning the time range 

 of the listed genera and species. Its truth is abundantly attested by com- 

 parison of such lists and statements published in the past fifty years or 

 more; and the necessity of seeking endorsement of the organic by the 

 physical criteria, in short, of great caution in deciding on a stratigraphic 

 correlation is emphasized each time a recurrence of species is established 

 or the vertical range of a genus is extended by new discoveries. To blindly 

 accept that a certain genus wherever found is diagnostic of a given epoch 

 or age is to risk impeding progress in exact correlation. At best such 

 practice is justifiable only on the ground of temporary convenience. 



(10) Indexical value of species in distinct provinces. — According to 

 views advanced in discussing the paleontological criteria of correlation 



