MIDDLE CAMBRIAN. 25 



5, 6, and 7 of PI. I and fig. 1 of PI. II is unbroken. It is partially repre- 

 sented by figs. 1, 2, 3, 4, 6, 7, and 8 of PI. I and figs. 1 and 7 of PL II. 



subumbreiia. — 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 (PL 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. 

 Tlnee 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 (a) 

 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 b is broken down at the axis, but its point of attachment is indi- 

 cated. In fig. 8a of PL I 6 radial ribs imite at the base of the central axis 

 and 3 are attached above (x, x, x). The corresponding exumbrella lobes 

 are shown at x, x, x, fig. 8. Transverse sections of such lobes are shown 

 in figs. 1, 6, and 14 of PL IV. 



interumbrella. — The 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 interunibrella 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 (PL I, fig. 7), 

 or at any point along down the axis to the base of the oral disk (PL II, 

 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 II, figs. 2-5). In connection with the rounded, radial ribs 

 of the subumbrella, such forms appear at first to be a different species, when 



