MIDDLE CAMBEIAN. 



9 



of Brandt's restoration of Rhizostomites (text fig. 1 8, A, p. 80) is strikingly 

 similar to that of Brooksetta alternate (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 medusae 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 medusa?. 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- 

 dusse, while the closed mouth and oral 

 arms suggest the Rhizostomse. The 

 accompanying diagrammatic restora- 

 tion of a vertical section of Brooksella 

 altemata (text fig. 1), when compared 

 with the vertical section of Cannorluza 

 connexa (text fig. 3, p. 11), illustrates 

 the ordinal relationship between them, 

 and one is almost inclined to refer the 

 Cambrian species to the Rhizostomse, 

 and thus place them in the same 

 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- 

 rhUa primordialis is considered by Dr. Haeckel to be the simplest and most 

 primitive form of all the known Rhizostomse. 1 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, 



Fig. 1. — Brooksella altemata. Central vertical section. 

 For description of figure, see text fig. 4, p. 28. 



'System der Mednsen, 1880, p. 565. 



2 System der Medusen, Atlas, 1879, fig. 2 of PI. XXXVI. 



