GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 573 



Tampa and Chipola pelecypods promises to be similar to that be- 

 tween the gastropods. No identical species of any significance has 

 been found, and except a single conspicuous element the entire 

 aspect of the fauna looks forward to the later Tertiary and Recent 

 rather than backward. The presence of Orthaulax, that bizarre 

 group so closely associated with the Oligocene of the southeast 

 coast and the Antilles, is the one strong band between the Chipola 

 and the later Oligocene faunas. This archaic type survived the 

 break at the close of the Tampa and continued in considerable 

 abundance throughout the Chipola, but no trace of it has been 

 found in the later formations. 



' ' The affinity between the Oak Grove and Chipola is much closer than 

 the percentage of identical species indicates. Only about 15 per cent 

 of the Chipola forms are common to the Oak Grove, although about 

 35 per cent of the Oak Grove forms are common to the Chipola. The 

 Chipola fauna is remarkably varied and includes two decidedly dis- 

 tinct facies and a third more obscurely differentiated assemblage. 

 The Oak Grove fauna, on the other hand, is much more uniform; it 

 includes fewer species and has a much larger relative number of 

 individuals. The facies of the Chipola fauna at the type exposure 

 on Chipola River is much more closely allied to the Oak Grove than 

 is the facies developed in the lower bed at Alum Bluff, which con- 

 tains a rather prominent brackish water element. The third assem- 

 blage, a marine fauna known only from Boynton Landing on Choc- 

 tawhatchee River, has a rather large number of peculiar species. 

 Except Orthaulax, the prominent genera of the Chipola fauna on the 

 Chipola River and those of the Oak Grove fauna are the same, and a 

 goodly percentage, probably the majority, of the prolific species of the 

 Oak Grove have closely related analogues in the Chipola fauna as 

 represented on Chipola River. The change following the Chipola 

 was apparently sufficient to exterminate the archaic types, together 

 with a large number of the newer forms. The hardier types, however, 

 survived and were apparently able to flourish with increased abun- 

 dance in the less densely populated waters of the Oak Grove." 



The Mollusca of this horizon are only remotely related to those of 

 the Tampa formation, which is the stratigraphic equivalent of the 

 upper part of the Chattahoochee formation, while they are closely 

 related to those of the next higher zone, the Oak Grove sand. Be- 

 cause of the faunal kinship and the stratigraphic intergradation of 

 the marl with the typical material of the Alum Bluff formation at 

 Alum Bluff, it is classified with the Oak Grove sand as a member of 

 the Alum Bluff formation. 



Berry has described the small flora obtained in the Alum Bluff 

 formation * in a paper by him already cited. The fossil plants at 



» U. S. Geol. Survey Prof. Paper 98 (E), pp. 41-59, pis. 7-10, 1918. 



