JURASSIC. dad 
This species and the following, R. lithographicus, were referred to the Rhizostomid 
because they have the center of the mid-field occupied by a regular and sharp cross- 
shaped figure, which is referable to the connate bases of the feeding arms, which are 
so characteristic of the rhizostomes. The complete lack of marginal tentacles, 
together with the division of the umbrella rim into numerous lobes, is also suggestive. 
The absence of the median spaces observed between the outer and inner ring of 
Acraspedites antiquus shows that the greatest diameter of the disk in R. admirandus 
and R, lithographicus was at the rim, as in the case of most of the Rhizostomide. 
It is especially noteworthy that small examples of R. admirandus have the same 
number of marginal lobes (as nearly as can be ascertained) as the largest specimens. 
The sharp and characteristic structure of the mid-field is explained completely 
and naturally on the supposition that the mouth opening is lacking, and that the four 
regular, equal rims of the cruciform furrow (a—c), which meet at right angles in the 
center, are the seam-like boundaries between the four connate bases of the four great 
feeding arms, which in rhizostomes hang down from the middle of the under surface 
of the disk, where in other medusz the mouth is situated. 
The four convex isosceles triangles are interpreted as the basal portions of the 
four great arms or so-called branches of the gastral trunk which hang down from the 
center of the umbrella cavity and correspond to the four mouth arms of other 
Acraspede. 
At the periphery of the mid-field each principal arm or trunk appears to bifur- 
cate, so that eight arms start from the periphery of the mid-field. Whether the eight 
arms which result in this way branch again, it is not possible to follow up, since the 
outlimes, which in the case of the four arms are so sharp in the mid-field, gradually 
disappear toward the exterior and are lost in the confused, irregular lines and depres- 
sions of the first concentric ring. However, this is probably not the case. Further- 
more, the unbroken periphery of the concentric ring indicates that no arms overlie 
it, and that, within, the eight arms were both simple and short. 
Of the four concentric rings which surround the mid-field, the first or deep ring 
(n) corresponds without doubt to the four genital cavities. This is shown as well by 
its position as by the irregular nature of its upper surface, and finally by its sug- 
gestive depression. Since this ring is the deepest portion of the whole impression, 
the medusa disk must here have been the thinnest, and this, in fact, corresponds 
completely to the circumstance that in many acraspedote meduse that portion of the 
disk in which the genital cavities lie is the thinnest portion of the umbrella. 
The second or four-part ring (p) can be regarded with great probability as the 
genitalia themselves, or a part of them. In fact, in some rhizostomes the sex organs 
lie on the outer periphery of the pouch-shaped genital cavities, in the form of elon- 
gated cylindrical rolls, which are concave toward the center. Each of the four eres- 
centic mounds is, then, to be regarded as a reproductive organ. The peculiar furrowed 
character of their upper surface tallies with this interpretation. The fine furrows on 
it do not go to form connected concentric rings, as do the regular circular furrows of 
the outer (muscle) ring (w), and can not therefore be referred to the ring muscles 
