403 



Costa Rica Miocene — Olsson 



Gatun Stage: Gatu7i, and Mt. Hope, C. Z, 

 Water Cay. 



Margarita Trail, C. R. 

 Genus DOSINIA, Scopoli 



Section DOSINIDEA, Dall 



Oosinia acetabulum Conrad Plate 31, figure I 



Artemis acetabuliivi Conrad, 1833. Fossils of the Tertiary Formations, 

 p. 20, pi. 6, fig. I. 



Dosi?zia acetabulum Bose, 1906, Boll, de Inst. Geol. de Mexico, num- 



ero 22, p. 81, pi. II, figs. 7, II. 

 Dosinia {Artetuis) cf. acetabulum Tou\a., 1908, Jahrb. der K-K, Geol. 



Reichsanstalt, Wien, vol. 58, p. 727, pi. 27, figs. 8, 8a. 



The Dosinia acetabulum has been recorded by Bose from 

 Mexico and by Toula from Panama, but their determinations 

 were based on imperfect material or casts. The Costa Rican 

 collection however contains a large series of finely preserved Do- 

 sinias which are unquestionably the Dosinia acetabulum Conrad 

 of the Chesapeake Miocene. 



The surface of the Costa Rican specimens are generally 

 neatly sculptured with even, concentric bands or ribbons which 

 on the middle of the shell disk average 2 mm in width. The 

 form of the shell is subcircular and like the typical D. acetabu- 

 lum of the Yorktown formation of Virginia and North Carolina. 

 The Costa Rican shells are equally as large as the Chesapeake 

 shell measuring in height 70 mm or more. 



The Dosinia acetabulum in the eastern United States, is a 

 most characteristic fossil of the Chesapeake Miocene, where its 

 range extends throughout most of the lower and upper Chesa- 

 peake formations. It is lacking from the Lower Miocene or the 

 Alum Bluff formation of Florida, as well as from the succeeding 

 Pliocene. In its range through the Chesakeake Miocene, it 

 gives rise to several well-marked varieties of which the broad 



