24 FOSSIL MEDUSA. 
2. The influence of the condition of preservation is readily seen in the 
form of the umbrella in distorted and broken specimens, but it is not so 
easily determined with relation to the presence or absence of the gelatinous 
matter that is so prominent in the umbrella of recent species of the Acras- 
peda. In some individuals it appears to be present (PI. I, figs. 1, 2, 3, 4), 
while in others the firm, supporting ectoderm appears to have shrunk so as 
to leave only a skeleton form of the umbrella (PI. I, fig. 7; Pl I, figs. 5 
and 6). Frequently the medusa is completely embedded in the siliceous 
matter of the nodule, and it is only by cutting sections that its form can be 
observed. The embedding may be entire (Pl. IV, figs. 10, 11, 13) or partial 
(Pl. UU, fies 2,09; Pl Vs ies Synonus)): 
The lobation of the umbrella is rarely, if ever, lost; it is the dominant 
character in all the specimens, and extends from or near the center to the 
margin and forms the lobate marginal border (PI. I, figs. 1, 2, 4), which is 
often deeply indented (PI. ITI, figs. 1, 5, 6). The lobes vary in number 
from 6 to 20, or more, and in form from broad, slightly rounded to narrow 
and strongly rounded. There is no regular sequence of 6, 8, 12, etc.; on 
the contrary, the irregular numbers 5 and 7 are largely represented (PI. I, 
figs. 4 and 5; Pl. III, fig. 1), and 6 and 8 are abundant. The sinus between 
the lobes may be merely a depressed line (PI. I, figs. 1, 2, 3), or it may cut 
through to the base, leaving only the central portion of the lobes attached 
to the umbrella disk (PI. 1, figs. 5, 7, 8; Pl. II, figs. 3, 5, and 6), or it may 
be irregular, varying in size and arrangement (Pl. I, figs. 7 and 8; Pl. I, 
fig. 5). In the former instances the umbrella is symmetrical and relatively 
smooth, while in the latter the surface is broken by the normal series and a 
secondary series of lobes that may be represented in an individual by a 
single narrow lobe projecting from between and beneath two of the upper 
lobes (PI. I, fig. 6, x), or by a much larger number, as seen in PI. I, fig. 1, 
where eight project from beneath the upper series. The variation in the 
latter group is considerable and is not reducible to any serial tabulation. 
The more regularly lobed individuals may be taken as the first type 
of the species (PI. I, figs. 1, 2, 3, 4), and the irregularly lobed as a variety 
(Pl. I, figs. 6 and 7; Pl II, figs. 1 and 3). So many gradations occur 
between the two extremes (fig. 4 of PI. I, and fig. 1 of Pl. ID), that I do not 
find it practicable to distinguish two species or even varieties. The passage 
from the typical forms (PI. I, figs. 1 to 4) to the variety represented by figs. 
