746 STUART WELLER 
ogy and Natural History of Indiana, Mr. S. A. Miller has illus- 
trated a single quadrant of a similar fossil from the Niagara lime- 
stone at St. Paul, Ind. A short description is published, but no 
name is given to it. - Mr. Miller states in his description that he 
has seen a similar fossil from the Niagara limestone near Chi- 
cago, circular in outline, and made up of four such segments as 
he illustrates from St. Paul. The fossil he had in mind is doubt- 
less one of those illustrated in the present paper. 
Since the fossil to which Hall gave the name Cryftodiscus is 
without doubt a form similar to those under discussion, his 
name will be used in their description, although it has never 
been properly published. 
No satisfactory explanation of the nature of Cryptodiscus has 
been given by either Hall or Miller. Hall’s statement with a 
query, that it is the calyx of a crinoidean can hardly be correct as 
the quadrangular symmetry is unlike that of the corresponding 
part of any crinoid. Miller states that some collectors have con- 
sidered the fossil to be the operculum of a coral, but he himself 
does not seem to regard this interpretation of it as the correct one. 
The genus Gontophyllum is the only coral possessing an operculum 
of four triangular plates, but Cryptodiscus, from the configuration 
of the disk, need not be compared with the operculum of Gonzo- 
phyllum. 
After a careful consideration I am led to believe that these 
peculiar fossils may be the remains of meduse. Though the mod- 
ern jellyfish are often of large size, they are singularly unfitted 
for preservation as fossils because of the entire absence of hard 
parts. Under favorable conditions, however, impressions of 
these organisms have been preserved which admit of syste- 
matic determination. Such impressions have been described 
from the Upper Jurassic and the Upper Cretaceous in Europe. 
Of a more questionable nature are the Cambrian fossils from 
Sweden and America which have been referred to the Meduse 
by Nathorst and Walcott. 
Although Cryptodiscus has in general the symmetry of the 
medusz, the idea of its being the mere impression of such a 
