22 FOSSIL MEDUSAE. 



brian forms seen on Pis. XXIV-XXVIII. The occurrence of such forms 

 as those shown on Pis. I-VIII in the same shale with a compressed specimen 

 like fig. 7 of PI. VIII is scarcely conceivable if they were siliceous or 

 calcareous sponges. Nearly all of the Lower Cambrian specimens are thin 

 films of slightly carbonaceous matter between the laminae of the slate. 

 Annelid trails and burrows in the same slates are usually compressed to a 

 thin film, but often they are preserved so as to show a round or oval trans- 

 verse section. Only in a few rare instances have the fossils referred to the 

 Medusae shown any convexity. When this occurs, as in fig. 2 of PI. XXIV, 

 and in fig. 1 of PI. XXV, it suggests a partially compressed medusa. Fig. 7 

 of PI. VIII could not have been compressed after the mud was hardened 

 into rock, as the fragments of trilobites and brachiopods in the shale show 

 no evidence of any considerable amount of compression. A few nodules 

 of chert associated with the Medusae cherts have large, well-preserved 

 casts of spicules of a hexactinellid sponge (Protospongia?) attached to 

 their outer surface, and in one instance buried in the body of the nodule. 



I have called attention to the possibility of the Middle Cambrian forms 

 being referred to the Spongiozoa in order to anticipate such suggestion and 

 to explain that it has been considered as one of the possibilities in determin- 

 ing the character of the remarkable fossils now under consideration. 



DESCRIPTION OF GENERA AND SPECIES. 



Order SCYPHOMEDUS^E (ACRASPEDA). 

 Suborder DISCO-MEDUSAE. 



Family BEOOKSELLID.E. 



f BROOKSELLA. 

 Genera . . J LAOTIEA. 



[daottloidites. 



Genus BROOKSELLA Walcott. 

 Broolcsella Walcott, 1896. Proc. U. S. Nat. Mus., Vol. XVIII (1895), p. 611. 



Discomedusae with a lobate umbrella, 6, 7, to 12 or more lobes; without 

 tentacles and without (?) central oral opening ; with a simple radial canal 

 in each lobe of the umbrella and each interradial lobe, when the latter are 



