MIDDLE CAMBRIAN. 9 
e 
of Brandt’s restoration of Rhizostomites (text fig. 18, A, p. 80) is strikingly 
similar to that of Brooksella alternata (text fig. 1, below), but the similarity 
is one of an ordinal or subordinal character. 
RELATION TO LIVING MEDUSA. 
It is not to be supposed that the Cambrian medusxe were similar to 
any living species, genus, or family. The great time interval and the 
changes that have taken place in the associated fauna lead us to look for 
only slight resemblance to the living forms. 
Nothing is known of the genitalia or sense organs of the Middle 
Cambrian fossil medusze. The general form of Brooksella is depressed 
discoidal, and the stomach is surrounded by radial canals. These characters 
enable us to place it in the Discome- 
dusee, while the closed mouth and oral 
arms suggest the Rhizostome. The 
accompanying diagrammatic restora- 
tion of a vertical section of Brooksella 
alternata (text fig. 1), when compared 
with the vertical section of Cannorhiza 
connexa (text fig. 8, p. 11), illustrates 
the ordinal relationship between them, 
and one is almost inclined to refer the 
Cambrian species to the Rhizostome, 
and thus place them in the same 
Fic. 1.—Brooksella alternata. Central vertical section. 
For description of figure, see text fig. 4, p. 28. 
family. The lack of knowledge of many critical features of the fossil form 
compels its reference to a distinct family. Among the living species, Archi- 
rhiza primordialis is considered by Dr. Haeckel to be the simplest and most 
primitive form of all the known Rhizostome.’ The disk-shaped umbrella; 
simple, undivided, almost cylindrical oral arms; individual marginal lobes ; 
simple form of canal system, all suggest the Middle Cambrian type much 
more closely than any other recent form. To enable the student to make 
direct comparisons, the view of the lower side of the medusa is reproduced 
from Haeckel (text fig. 2, p. 10).2. The canal system is more complex than 
in Brooksella; and there are other marked differences. It is instructive, 
‘System der Mednsen, 1880, p. 565. 
* System der Medusen, Atlas, 1879, fig. 2 of Pl. XXXVI. 
