FOSSIL MEDUSA. 



By Charles D. Walcott. 



INTRODUCTION. 



The occurrence of impressions of medusae on the Jurassic lithographic 

 limestones of Solenhofen has long been known and is referred to in nearly 

 all comprehensive works on paleontology. In 1881 Dr. Nathorst called 

 attention to certain problematic fossils from the Lower Cambrian rocks of 

 Sweden, which he regarded as casts of the impressions of the lower side 

 of medusae that had been left on the soft mud by the tide or had been 

 thrown up by the waves; others were thought to be casts of the gastric 

 cavity of a medusa having a large mouth opening and a large gastric 

 cavity. In 1891 I called attention to the Lower Cambrian Dactyloidites 

 asteroides, suggesting that it might be the impression of the mouth and gastric 

 cavity of a medusa. In the last four years I have studied, during short 

 intervals of time taken from administrative work, large collections of 

 medusae from the Middle Cambrian rocks of the Coosa Valley, Alabama. 

 The original plan was to incorporate the results in a monograph on the 

 Middle Cambrian fauna; but now that the study has included the Lower 

 Cambrian and Jurassic types, it seems advisable to publish a separate 

 memoir on the subject of fossil medusae, in order to place before students a 

 full review of the subject. 



The Middle Cambrian medusae will first be described, and then, in 

 order, the Lower Cambrian of the United States and of Sweden and 

 Bohemia, and the Jurassic of Bavaria. 



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