52 FOSSIL MEDUS 2. 
certainty. * * * The various forms under which Medusites lindstrémi occurs may 
properly be classified as follows: 
(a) Imprint of lower side: 
(1) Shows only irregular impressions of tentacles! (?). 
(2) Like (1), but has besides in the middle a pyramidal filling of the mouth 
opening, at times also with filled-up genital apertures; imprint of tentacles 
(2) often lacking. [See fig. 1, Pl. XXIX, of this memoir.] 
(3) Like (2), but corner of pyramid continues outward in the filling of the arm 
grooves. 
(4) Imprint of arms or filling of their grooves, either four- or five-parted, at 
times with imprint of genital apertures. 
(5) Filling of arm grooves (in a five-parted specimen), together with imprint of 
the whole mass of the body. 
(b) Closed casts of gastric cavity: 
(6) Pyramidal or hemispheric, free, four- or five-parted, with base four-parted or 
roundly five parted or round, sides bounded by four or five sharp edges, 
and sides between them either flat, convex, or concave, at times with pro- 
jecting parts, corresponding to the filled-up genital apertures. 
Here may properly be mentioned acircumstance which harmonizes very 
well with the reference of those fossils to Medusze, and gives external con- 
firmation to the correctness of such reference—that is, the great difference 
in size among the various specimens. While the smallest specimens have 
a base of only about 12™™, the diameter of the largest is at times 60™™. 
This is precisely what is seen in meduse, the greatest differences being 
found in various individuals of the same swarm. 
(7) A free biconvex cross, composed as it were of two halves laid one on the 
other. It is uncertain to which of the two principal groups this form 
should be referred. If it is really the filling of a gastric cavity, it must 
no doubt be derived from a special species. Otherwise, it would be the 
filled-up grooves of the arms, together with a mass accumulated by means 
of concretion, a theory which seems to be contradicted by the little round 
impression in the middle. Itis to be hoped that further discoveries will 
settle this question, as well as the question whether Medusites lindstrémi 
comprises only one or several species. 
No. 2 of class ais illustrated by fig. 1 of Pl. XXIX, and the close 
casts of the gastric cavity (class b) by the figures of Pls. XXVIII and XXIX. 
The former is considered by Dr. Nathorst to be the cast of the lower sur- 
face of the medusa, the central pyramid and four radiating ridges being 
the cast of the opening of the mouth. 
‘This impression, as in form 3, at times extends to the mouth pyramid, and the question might 
thus be asked whether it might not rather be regarded as derived from mouth curtains or sex curtains. 
On the other hand, however, the tentacles might have accidentally been bent toward the center. 
