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 Rhizostomidse 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. 1 



CLASSIFICATION. 



Dr Haeckel's classification is scattered through his publications on the 

 fossil medusas 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- 

 zostomese. It approaches on the one hand the Rhizostomidpe, through the 

 subgenital opercula and the muscular system, and on the other the Cram- 

 bessidfe, through the families Colostylida? and Leptobrachidae, on account 

 of the broad, short arm disk and the hypothetically long, thin arms. 



The similarity of the Jurassic Rhizostomse to the Crambessida?, clearly 

 enough expressed in the structure of the body, especially the mouth disk, is 

 noteworthv, in so far as the latter are the most aberrant of all living families 

 of medusa?. 



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 medusas. 2 



'Brandt, Ueber fossile Medusen: Mem. Acad. imp. sci. St. Pe'tersbourg, 7th series. Vol. XVI, 

 No. 11, pp. 13, 14. 



2 Ueber neue Examplare von jurassischen Medusen: Abhandl. Math.-phys. Classe Konigl. bay- 

 erisehen Akad. Wiss., Vol. XV, 1886, p. 165. 



