68 FOSSIL MEDUS. 
represent the circumference of the head and the arms of a cephalopod. 
Important observations were made on the classification of the Jurassic 
medusz that will be referred to again. 
ORIGIN OF THE IMPRESSIONS OF MEDUS&. 
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 meduse, it is very improbable that one should become embedded 
and petrified on the bottom of the high sea. The Jurassic fossil meduse 
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 
medusz, 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 
