68 FOSSIL MEDUSA. 



represent the circumference of the head and the arms of a cephalopod. 

 Important observations were made on the classification of the Jurassic 

 medusae that will be referred to again. 



ORIGIN OP THE IMPRESSIONS OF MEDUSAE. 



Dr. Haeckel advanced the view that the impressions were secondary and 

 not direct. Dr. Brandt, on the contrary, regards them as direct impressions, 

 and in this Dr. von Ammon agrees with him. Dr. Brandt thinks that on 

 account of the low specific gravity and the well-known physical constitu- 

 tion of medusae, it is very improbable that one should become embedded 

 and petrified on the bottom of the high sea. The Jurassic fossil medusae 

 therefore belong to stranded individuals. They did not lie on their side, as 

 Leptobrachites trigonobrachius did, but were spread out on their under or oral 

 surface. The fluid calcareous slime filled the under surface of the oral disk 

 and the umbrella almost completely. The small quantity of air or water 

 which might in the process have become caught under the medusa-bell 

 would become forced into the most excavated zone of the umbrella, and 

 there have occasioned the existence of the so-called smooth ring through 

 which the impressions are interrupted. Through the oval apertures the 

 limy ooze forced its way into the four genital cavities. Although the latter 

 must have been collapsed by reason of the hypothetical position of the 

 medusa?, the intrusive mass succeeded in taking impressions of the covers 

 of the genital cavities, the saddle-shaped plats. That these last appear as 

 raised positions on the mid-field is sure indication for the interpretation of 

 the fossils as impressions in the narrower sense of the word. The ccelenteric 

 central cavity remained unfilled, except, perhaps, as shown in the fossils by 

 a few irregular attached pieces of limestone, where the ooze may have 

 pressed in here and there, either through a slight rupture or through a still 

 open portion of the mouth. Concerning the structure of the floor of the 

 central cavity, therefore, no information could, under these circumstances, 

 reach us. 



Only by the mode of fossilization just described is it explicable that 

 no impressions of the oral arms exist, namely, that they could very well lie 

 under the surface of the slab, inside it, and there have left their impressions. 

 Against the probable objection that at least the basal portions of the arms 



