422 O. Meyer — Genealogy and Age of the Sj?ecies 



typical Jackson bed ? If two of the beds were connected di- 

 rect]} 7 , for instance, by a river, and we could follow the strata 

 foot by foot, then stratigraphy alone could determine their rel- 

 ative age without an examination of fossils. As this is not 

 possible there remains only to determine strata at a distance 

 from a typical localitjr as belonging positively to a certain 

 period, and then to study the strati graphical relations to each 

 other or to one of the typical localities. Can we use lithologi- 

 cal characters in the determination, of a stratum, whether it was 

 deposited in the Claibornian, Jacksonian or Yicksburgian time? 

 Professor Smith and Mr. Aldrich seem inclined to use litholog- 

 ical characters ; but I cannot attribute to them any value, and 

 Professor Hilgard seems to be of the same opinion, for he says 

 (II, p. 30): "A great deal of the obscurity in which the relative 

 age of the Southwestern Tertiary has been involved, is owing to 

 too great a reliance placed by most observers on lithological 

 characters, differences as well as resemblances," and (II, p. 31) 

 " Nowhere has the geologist more need of divesting himself of 

 reliance upon lithological characters, than in the study of the 

 Mississippi Eocene." So we see that we are led to the exami- 

 nation of fossils, and I have only to show now that this exami- 

 nation must be " competent and careful. 1 ' 



Certain fossils may have lived in a single one of three periods 

 only, as far at least as the region in question is concerned. 

 Such fossils are then characteristic of this era. They may be 

 designated here as a fossils. I am not able to name with certainty 

 any example of such a fossil in Alabama and Mississippi. 



Another class of fossils, here called /? fossils, occur in two or 

 more of these beds, so nearly alike that no differences are recog- 

 nized and known. For instance, I am unable to discriminate 

 between Cytherea minima Lea from Jackson and Claiborne, be- 

 tween Alveinus minutus Com*, from Yicksburg, Jackson and 

 Claiborne, and nobody else has as yet published a recognized 

 difference. 



A third class of fossils, y fossils, is numerous in the Southern 

 Old-tertiary. It embraces such fossils as occur in two or three 

 beds in forms, which, though similar, are in some respects more 

 or less different from each other; these differences being in 

 some cases so slight as to be apparent only on the closest ex- 

 amination ; in other cases more apparent, and sometimes rather 

 great. For instance, Venericardia planicosta in Jackson and 

 Claiborne differ, but the difference is so slight, that I have not 

 applied varietal names. In other cases I consider the differ- 

 ences sufficient to justify the use of a varietal or even a specific 

 name. For examples, see Part I of this essay. 



A fourth class, called o fossils, are such as have been hitherto 

 found in one of the three localities only, with no similar forms 



