JURASSIC. 85 
numerous openings existed on the strong arms which hung down from the center of 
the inferior surface of the disk. Such arms, which were connate at their bases, appear 
to have been in the present case 8. 
As concerns the three concentrie ring fields which surround the mid-field in R. 
lithographicus, the first or innermost of these, which we have here called the deep ring, 
on account of its strong depression, can be here likewise interpreted as the genital 
field, since in it, without doubt, the genital cavities were located, and in them the sex 
organs. It is, however, not possible to pick out clear and certain forms from the unin- 
telligible confusion of fine lines and furrows which traverse the whole deep ring; and, 
especially, of the sickle-shaped genital mounds, which in R. admirandus combine to 
form the four-fold ring, there is no trace. 
The smooth ring (s) is similar in every way to the smooth ring of R. admirandus. 
The furrowed ring —The number of ring muscles of the subumbrella is only half as 
great (20) in R. lithographicus as in R. admirandus. The eight greater indentations 
of the disk rim, four radial (y) and four interradial (7), in which the rim bodies were 
situated and by which eight bow-shaped lobes were formed, are likewise less distinct 
here than in R. admirandus. Further, these are made up of smaller lobes (2), which 
are formed by short radial furrows in the rim, and whose number may be reckoned at 
about 112. 
On the whole, we can maintain that R. lithographicus is an acraspedote (dis- 
cophorous) or higher medusa, and very probably belongs to the suborder of the 
Rhizostome, but are not in position to determine definitely its family, although it is 
probably of the Rhizostomidee. 
It was Dr. Haeckel’s original purpose to refer all species of fossil 
medusze to the genus Medusites, or, in case they could be referred to one 
of the two principal divisions of the medusze, he proposed to use Acraspe- 
dites or Craspedonites. This was made with the belief that zoologists would 
never be in position to determine accurately the family of fossil medusee. 
The discovery, however, of . admirandus proved that he was mistaken; 
so he proposed Rhizostomites to receive the new forms, stating at the same 
time that if anyone was unwilling to accept his conclusions and evidence, 
the fossils might be placed in the genus Acraspedites or even Medusites. 
In his review of the fossil medusze of the Jura, in 1869,' Dr. Haeckel 
states that perhaps this species is only a young example of R. admirandus, 
or, it may be, a quite different rhizostome. The eight three-cornered arching 
arms of the mid-field, which were interpreted in the original description as 
the basal portions of eight powerful arms, are perhaps with greater proba- 
bility to be regarded, as in R. admirandus, as four arm bases with four genital 
cavities alternating with them. 
1 Zeitschr. fiir wiss. Zoologie, Vol. XIX, 1869, p. 558. 
