26 FOSSIL MEDUS&. 
compared with such regular forms as are represented by figs. 1 and 3 of 
Pl. I. The larger proportion of the specimens possessing the latter char- 
acters are flattened on the lower side so as to form a compressed disk, but 
a few specimens retain some of the original fullness and have a transverse 
spheroidal outline. 
Umbrella corona— The line of the corona furrow is suggested by the ring 
about the central disk (PI. I, figs. 1, 2, and 3). It is quite probable that 
this is correct, as the sutures between the exumbrella lobes cut back to the 
disk in nearly all, if not all, the specimens (PI. I, figs 1, 2, 3; Pl. II, figs. 
1, la, 5, 6). The ring is not always present, but it usually is in the more 
perfect specimens, where the distinct exumbrella lobation does not continue 
to the center of the disk. 
The umbrella margin is deeply lobed, following the rim of the exum- 
brella lobes. In such specimens as are represented by fig. 5 of Pl. II and 
figs. 3, 6, ete., of Pl. III, it could have been little more than the irregular 
rim of the various exumbrella and interumbrella lobes. The presence of 
organs of sense on the margin is not known from the fossil specimens. It 
is only by the relations of the species to the recent Discomedusze that we 
may assume that they were present. Haeckel says:* 
The umbrella margin is the most important part of the neuroderma! system in 
all meduse, both morphologically and physiologically, as in it the most important 
animal organs—organs of sense, nerves, and muscles—attain their highest develop- 
ment. The central part of the nervous system and the tentacles especially are always 
originally situated in the umbrella margin. The umbrella margin is also of great 
importance for classification, as it is chiefly on it that the variations of formation 
appear which lead to the distinction of genera and species. In fact, the distinction 
and nomenclature of the two principal divisions of the class Meduse, of the two sec- 
tions Craspedota and Acraspeda, are taken from the umbrella margin, which presents 
important and striking diversities in the two sections. The ‘“‘velum” is characteristic 
of the former, the ‘‘ lobe corona” of the latter. 
In Brooksella alternata and associated meduse no tentacles have been 
observed. Organs of sense, touch, smell, vision, and hearing probably 
existed. We can imagine the existence of sense clubs from analogy with 
recent Discomeduse, but they are not preserved in the fossil state. All 
such organs appear to have been destroyed in the process of fossilization. 
Gastrovascular system.—In all meduse the gastrovascular system or intestinal system 
is divided first of all into two principal sections, a central and a peripheral part. For 
‘Report on the deep-sea meduse: Voyage of H. M.S. Challenger; Zoology, Vol. IV, 1882, p. xlii. 
