LOWER CAMBRIAN. 45 
XXVII were given their present outlines. A medusa resting on its back 
on the bottom and covered with a deposit of fine mud would be pressed 
out, and as the pressure increased there would be a tendency to rupture 
it. In the examples cited, and in fig. 1 of Pl. XXIV and fig. a of Pl. 
XXVI, the rupture appears to have been in the exumbrella, and the 
broad ends of the exumbrella lobes were left attached to the narrow sub- 
umbrella lobes so as to give them the appearance of having had a bulb 
attached to their outer end. The fact that the exumbrella lobes have a 
central canal accounts for the traces of openings in the broad terminations 
of the radiating lobes in fig. 1 of Pl. XXIV and fig. 6 of Pl. XXVII. In 
fig. 1 of Pl. XXIV it appears as though each of the entire exumbrella lobes 
had been drawn or pressed out and flattened. An example of the drawing 
out of the subumbrella lobes in one of the flint nodules from the Middle 
Cambrian is shown by fig. 2 of Pl. XXII, and of the flattening of the outer 
portion of the exumbrella lobes while attached to their subumbrella lobes, 
by figs. 5 and 6 of Pl. XXII. This is also shown for Brooksella alternata by 
figs. 1 and la of Pl. III, and many other specimens in the collection. It 
may readily be conceived that the flattening of a specimen, like that shown 
by fig. 3a of Pl. IX, would give a series of narrow, radiating lobes with 
broad, spatulate outer ends. In fig. 5 of Pl. XXVIII the entire medusa is 
flattened to a film that spreads out on the slate in strong contrast with the 
narrow lobes of fig. 3 of Pl. XXV. The interpretation is that the broader 
exumbrella lobes were pressed out to form the lighter or thinner portion, 
and the narrow subumbrella lobes to form the thicker, dark parts of the 
radiating lobes. 
Whether D. asteroides was reproduced by fission is not determined. It 
is very strongly suggested by the specimen illustrated by fig. 2 of PI. 
XXV, where there appear to be two individuals connected only by a 
single lobe—much as in the example of Laotira cambria, fig. 2 of Pl. XIX. 
The specimens illustrated on Pls. XXIV-—XXVIII are considered to be 
the best examples of the species in the collection. There are many speci- 
mens compressed and distorted in various ways, as would be the case if a 
comparatively soft body, like a medusa of the rhizostomean type, were 
compressed between laminz of mud. From the condition of preservation 
of this species it seems probable that the medusz were either less firm in 
structure, and hence offered less resistance to pressure, or more macerated 
