92 FOSSIL MEDUSZ. 
brian forms seen on Pls. XXIV-XXVIII. The occurrence of such forms 
as those shown on Pls. I-VIII in the same shale with a compressed specimen 
like fig. 7 of Pl. 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 laminz 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 
Medusz shown any convexity. When this occurs, as in fig. 2 of Pl. XXIV, 
and in fig. 1 of Pl. XXV, it suggests a partially compressed medusa. Fig. 7 
of Pl. 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 Medusz 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. 
T 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 SCYPHOMEDUS4: (ACRASPEDA). 
Suborder DISCOMEDUSZE. 
Family BROOKSELLID. 
BROOKSELLA. 
Genera .. < LAOTIRA. 
DACTYLOIDITES. 
Genus BROOKSELLA Walcott. 
Brooksella Walcott, 1896. Proc. U. S. Nat. Mus., Vol. XVIII (1895), p. 611. 
Discomedusze 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 
