MONOGRAPH OF THE MOLLUSCAN FAUNA OF THE 

 OETHAULAX PUGNAX ZONE OF THE OLIGOCENE OF 

 TAMPA, FLORIDA. 



By William Healey Dall, 



Curator, Division of Molluslcs, United States National Museum. 



INTRODUCTION. 



In the vicinity of Tampa Bay, Florida, and especially on the 

 northwestern shores of the bay, near Ballast Point, are found cer- 

 tain limestones more or less mingled with layers of clay, marl, and 

 chert, with residual sands and so-called " fuller's earth." A particu- 

 lar stratum which crops out near high-water mark at Ballast Point 

 is extremely fossiliferous. In the cherty portions the calcareous 

 matter of the fossils has disappeared through solution, and they are 

 represented chiefly by molds, from which casts may be made with 

 gutta-percha or other plastic material, so that the character of the 

 fossils can be determined. In the marly or clayey parts of this 

 deposit the fossils have also largely disappeared, but natural casts 

 in pure silex have replaced them. In the portions of the bed which 

 retain the character of limestone the fossils remain more or less 

 intact, but are difficult to work out on account of the hard, tough 

 character of the matrix. Rock excavated by dredges in deepening the 

 channel off Ballast Point, much of which has been dumped on the 

 adjacent beaches, is of the same character as the limestone above 

 tide marks in which the fossils remain calcareous. In that portion 

 of the bed in which silicification has been most active, besides the 

 shells exquisitely reproduced in silica, either translucent or of vari- 

 ous shades of brown, also occur silicified corals, some of them in 

 masses of considerable size. These have a geodic form in many 

 cases, the exterior of the coral head being reproduced, often with 

 great perfection of detail, while the interior is hollow, with its walls 

 covered with brilliant crystals of quartz, often presenting a re- 

 markable coloration in various shades of brown, red, blue, or yellow. 

 These having attracted the attention of visitors, were for years 

 collected by dealers in local curios for sale to tourists. The layers 

 54907°— Bull, 90—15 1 1 



