52 FOSSIL M EDITS JE. 



certainty. * * * The various forms under which Medusites lindstromi occurs may 

 properly be classified as follows: 



(a) Imprint of lower side : 



(1) Shows ouly irregular impressions of tentacles 1 (?). 



(2) Like (1), but has besides in the middle a pyramidal filling of the month 



opening, at times also with filled-up genital apertures ; imprint of tentacles 

 (?) often lacking. [See fig. 1, PI. 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 a circumstance which harmonizes very 

 well with the reference of those fossils to Medusae, 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 medusa', 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. It is to be hoped that further discoveries will 

 settle this question, as well as the question whether Medusites lindstromi 

 comprises only one or several species. 



No. 2 of class a is illustrated by fig. 1 of PI. XXIX, and the close 

 casts of the gastric cavity (class V) by the figures of Pis. 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. 



1 This impression, as iu form 3, at times extends to the mouth pyramid, and the <|uestion might 

 thus be asked whether it might not rather he regarded as derived from mouth curtains or sex curtains. 

 On the other hand, however, the tentacles might have accidentally been bent toward the center. 



