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 PI. XXIV and fig. a of PI. 

 XXVI, the rapture 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 PI. XXIV and fig. 6 of PI. XXVII. In 

 fig. 1 of PI. XXIV it appears as though each of the entire exumbrella lobes 

 had been drawn or pi-essed 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 PI. 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 PI. XXII. This is also shown for BrookseJ/a cdternata by 

 figs. 1 and la of PI. Ill, 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 PI. IX, would give a series of narrow, radiating lobes with 

 broad, spatulate outer ends. In fig. 5 of PI. 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 PI. 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 PI. XIX. 



The specimens illustrated on Pis. 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 laminae of mud. From the condition of preservation 

 of this species it seems probable that the medusa? were either less firm in 

 structure, and hence offered less resistance to pressure, or more macerated 



