JURASSIC. 69 
must have left traces, the following reply can be made: In view of the slight 
material consistency of the medusa body, the arm bases may find sufficient 
expression in the capillary division area, which is present between the point 
of projection of the arms and the irregular accretions which are situated at 
the periphery and which represent the tentacles. If it is asked why it was 
not possible, then, for the arms to be thrown up laterally and so impressed 
upon the fossil, it can be replied that the arms may have been quite short, 
or even, as is so often the case among Rhizostomidz of the present day, 
broken off during life. 
It is evident that the counter impression could be formed only when 
the umbrella and oral disk had dried up or decayed away, and the impression 
had had time to become hard.’ 
CLASSIFICATION. 
Dr Haeckel’s classification is scattered through his publications on the 
fossil medusze and is mentioned in connection with the descriptions of the 
species. Dr. von Ammon studied the various genera and species very 
thoroughly, and I shall follow his classification in this memoir. He creates 
for the reception of the Jurassic rhizostomites the extinct family Lithorhi- 
zostomez. It approaches on the one hand the Rhizostomidz, through the 
subgenital opercula and the muscular system, and on the other the Cram- 
bessidze, through the families Colostylidze and Leptobrachide, on account 
of the broad, short arm disk and the hypothetically long, thin arms. 
The similarity of the Jurassic Rhizostome to the Crambesside, clearly 
enough expressed in the structure of the body, especially the mouth disk, is 
noteworthy, in so far as the latter are the most aberrant of all living families 
of medusee. 
In view of the circumstances mentioned, it seems justifiable to consider 
the Jurassic rhizostomites as generalized types whose characters are to-day 
divided among the different families of medusze.* 
‘Brandt, Ueber fossile Medusen: Mém. Acad. imp. sci. St. Pétersbourg, 7th series, Vol. XVI, 
No. 11, pp. 13, 14. 
?Ueber neue Examplare von jurassischen Medusen: Abhandl. Math.-phys. Classe Kénigl. bay- 
erischen Akad, Wiss., Vol. XV, 1886, p. 165. 
