84 FOSSIL MEDUSAE. 



In comparing this species with B. admirandus, Dr. Haeckel says: 1 



At first sight the disk of Rhizostomites lithographicus is nearly like that of R. 

 admirandus, especially iu the structure of a strongly convex, octagonal mid-field, 

 which is surrounded concentrically by several ring fields. The mid-field is not, as iu 

 R. admirandus, divided by two bifurcating lines crossing at right angles in the center 

 into four alternating pairs of convex and concave isosceles triangles; at least, a struc- 

 ture corresponding to this can not with certainty be made out. On the other hand, 

 the mid-field is divided by eight radial lines into eight triangular fields, but these eight 

 areas appear to be pretty nearly of a size, so that alternately large and small triangles 

 can not be distinguished. Nor are the equal legs of the eight alternating isosceles 

 triangles bent. They are, on the contrary, almost straight lines. 



The inner or deep ring of R. lithographicus is traversed by furrows, ribs, eleva- 

 tions, and depressions according to no system which can be recognized. 



Of the four-part ring, which in R. admirandus, in the form of four crescentic 

 rolls meeting in four points, surrounds the inner or deep ring, there is no trace. They 

 appear to be lost in the outer portion of the inner ring. 



The middle or smooth ring, which here immediately surrounds the deep ring, 

 appears as a quite smooth, narrow band, everywhere of equal breadth, which, as well 

 on its outer as its inner margin (g), is sharply differentiated from both inclosing rings. 



The furrowed ring, here as in R. admirandus, is traversed by numerous fine, con- 

 centric furrows, which, however, are shallower and somewhat blunter than in the 

 latter. One can count only 20 instead of 40 in the breadth of the ring. 



The external portion of the furrowed ring is, in R. lithographicus, essentially 

 distinguished from that of Rhizostomites admirandus, in that the radial furrows, which 

 in the latter are so plainly impressed and divide the rim into such distinct lobes, in 

 the former are recognized only as quite weak and shallow notches in the disk edge. 

 The number of short lobes which they form may have amounted to about 112 in an 

 uninjured specimen, since there appear to be 28 of them to one quadrant. Besides 

 these small marginal lobes four larger, principal lobes seem to be indicated by four 

 deeper indentations, in which the sense organs probably lay. 



The inadequate definition of the mid-field, and especially of its center, which 

 admits of no certain conclusions as to the structure of the gastro- vascular system of 

 the medusa (R. lithographicus), renders it impossible to determine with certainty the 

 significance of the various structures and the affinities of the form itself. The four 

 intersecting central lines, which are so sharply impressed in R. admirandus, indi- 

 cating the undoubted rhizostomous nature of the specimen, are here lacking or are 

 indeterminable. On the other hand, the eight radial furrows, which divide the mid- 

 field into eight triangular areas, seem to have been connected in the center. Also, 

 the simple, large, individual and smooth mid-field, which is so plainly expressed in 

 Medusites antiquus and M. deperditus, and represents beyond a doubt the mouth and 

 gastral cavity, is lacking. Under these circumstances, it is most probable that R. 

 lithographicus, too, is a rhizostome; that it lacks a central mouth, and that, instead. 



1 Neues Jabibuoh filr Min., Geol. und Pal., Vol. VI, 1866, pp. 286-288. 



