MIDDLE CAMBRIAN. 25 
5, 6, and 7 of Pl. I and fig. 1 of Pl. Il is unbroken. It is partially repre- 
sented by figs. 1, 2, 3, 4, 6, 7, and 8 of Pl. I and figs. 1 and 7 of PI. I. 
Subumbretla— The subumbrella, exclusive of the oral disk and its append- 
ages, varies to nearly as great a degree as the exumbrella. Strong ridges or 
ribs radiate from the center to each of the principal lobes of the exumbrella; 
and sometimes the lobes separate above, so that there is little more than 
the central umbrella disk with a series of attached plates, like broad spokes 
in a wheel (PI. I, figs. 5, 5a; Pl. III, figs. 5 and 6). 
In addition to the main radial ribs that connect the principal lobes of 
the exumbrella with the body of the umbrella disk or axis, there is a vary- 
ing number of secondary, minor ribs that are connected with the smaller 
secondary lobes (interumbrella) that project from between the principal 
lobes and the radial subumbrella ribs. In fig. 6 of Pl. III the 8 regular 
radial ribs are present that correspond to the 8 lobes of the exumbrella. 
Three of the ribs are represented by their broken bases next to the central 
axis. In fig. 4 of Pl. I there are 7 exumbrella lobes and one extra rib (@) 
that was attached to the interlobe. At x (fig. 4a of Pl. I) an interumbrella 
lobe appears that is not attached to the central axis by a subumbrella rib. 
The rib } is broken down at the axis, but its point of attachment is indi- 
cated. In fig. 8a of Pl. I 6 radial ribs unite at the base of the central axis 
and 3 are attached above (a, xz, v). The corresponding exumbrella lobes 
are shown at 2, 2, x, fig. 8. Transverse sections of such lobes are shown 
in figs. 1, 6, and 14 of Pl. IV. 
Interumbrella— I'he interlobes are attached to the central axis, between the 
plane of the exumbrella and subumbrella lobes. A good illustration is shown 
by fig. 5 of Pl. III. In this, one of the interlobes does not appear to have 
any connecting subumbrella rib. A disconnected interumbrella lobe with 
its included radial canal, corresponding to the radial canal in each of the 
exumbrella lobes, may be attached to the umbrella disk or axis nearly 
on the plane of the dorsal surface of the exumbrella, x, x, x (PI. I, fig. 7), 
or at any point along down the axis to the base of the oral disk (Pl. I, 
figs. 2, 3, 4b). This results in a very complex structure, and instead of a 
simple symmetrical medusa, an irregularly lobed body results, with round, 
appendage-like lobes projecting out in various directions from the central 
disk or axis (Pl. I, figs. 2-5). In connection with the rounded, radial ribs 
of the subumbrella, such forms appear at first to be a different species, when 
