in the Southern Old-tertiary. 423 



in the other beds, with which they might be confounded. For 

 instance, Area Mississippiensis Conr. in Yicksburg. 



Having made this classification, we ask what is the value of 

 the fossils in each of these groups for determining the age of a 

 newly found bed in the region in. question. The ft fossils are 

 evidently useless for this purpose. For instance, Gytherea min- 

 ima Lea is of no use in distinguishing a Claibornian from a 

 Jacksonian bed, although this fossil, as long as it is not found 

 in Yicksburg, may be considered a 8 fossil in reference to the 

 Yicksburgian. 



This argument seems simple, and yet my emphasizing it ap- 

 pears to have been in vain. I had. to remonstrate in Part II of 

 this essay (p. 63) against Lyell's using, to demonstrate a Claibor- 

 nian age, fossils which he himself cites on the same page as occur 

 ring in Jackson and Yicksburg. Nevertheless, Mr. Aldrich and 

 Professor Smith cite (p. 306 and p. 274) fossils, as for instance, 

 Rostellaria velata Conr., Pecten DeshaysiljvSL, Turbinolia Maclurii 

 Lea, Dentolium thalloides Conr., Astarte sulcata Lea, Melongena 

 alveata Conr., Infundibulum trochiformis, Turritella lineata, 

 found in Jackson as well as in Claiborne, to demonstrate the 

 Claibornian age of a bed. Hilgard uses on p. 269 the genus 

 Orbitoides to distinguish from the Jacksonian a bed in Louis- 

 iana as Yicksburgian, while he himself has determined fourteen 

 years ago both known species of Orbitoides, and a new one be- 

 sides, as occurring in the Jacksonian strata of the same State 

 (See Hopkins's Second Annual Eeport of the Geol. Surv. of 

 Louisiana. New Orleans, 1871 ; pp. 11, 12, 13.) 



A single a fossil, however, would characterize a bed at once. 

 But what are a. fossils ? They may exist among the d fossils, 

 but they must not be confounded with them, d fossils, though 

 hitherto found in only one bed, are constantly being found on 

 each new examination, in the other beds. I have myself found 

 a large number of species at Jackson, which were known before 

 only from Yicksburg or Claiborne. Venericardia rotunda Lea, 

 and the allied form V. diversidentata Meyer have been known 

 before only from Claiborne and Jackson. Neither Conrad, nor 

 Hilgard, nor I have found this type in Yicksburg. So we 

 might have been tempted to consider this form as a good d 

 fossil in reference to Yicksburg. Now Mr. Aldrich says (his 

 article, p. 308) that he has it from Yicksburg. There is no end 

 of this. In the most thoroughly explored Tertiary localities in 

 Germany, almost every new examination extends the range of 

 known species, beside bringing to light new ones. There is 

 another reason why we cannot designate any fossil as a fossil. 

 We cannot expect that all the species that lived during the 

 Yicksburgian era should be represented at the locality at Yicks- 

 burg. During the same time very different species may have 



