MIDDLE CAMBRIAN. 21 
sponge is yet known in which the spicular skeleton retains its original 
transparent hyaline condition; the silica in all now presents numerous gra- 
dations between the amorphous, or colloid, and the crystalline state. It 
also frequently occurs that the silica has been partially or entirely dis- 
solved and replaced by calcite, glauconite, or other minerals, or entirely 
removed, leaving the empty cast of the spicule.” 
The existence of hexactinellid sponges in the Middle Cambrian sea 
is proved by the presence of casts of characteristic spicules on the exterior 
of a few nodules, and in one instance a nodule appears to have had many 
of the spicules as a nucleus. It is very probable that these few spicules 
are all that remains of an extensive growth of siliceous sponges, the great 
mass of the spicules of which disappeared by solution prior to the consoli- 
dation of the sediment. 
RELATION TO THE SPONGES. 
Early in my study of the Middle Cambrian fossils now referred to the 
Medusz, the question came up whether they were not a peculiar form of 
siliceous sponge. The same question has also been suggested by others 
when first looking at the specimens. After the preliminary study the 
material was laid aside for nearly two years before the final examination 
was made. In both the conclusion was that the fossils are the remains of 
meduse. The points of resemblance to the sponges are found in making 
comparison of the exterior form with Hallirhoa costata Lamx.’ and Brachio- 
spongia digitata Owen,‘ and of the interior canals with those of B. digitata. 
The resemblances, however, are of a superficial character. 
An examination of a large number of thin sections has failed to show 
any traces of spicule. But this is not conclusive, as the spicules, if present, 
might have been destroyed in the process of mineralization. In addition 
to the form and organization of the medusz, the most conclusive evidence 
to my mind that the forms under consideration are not sponges is the pres- 
ence of compressed specimens in the shale where only a slightly carbona- 
ceous film remains, as shown by fig. 7 of Pl. VIII and the Lower Cam- 
1T find that the term ‘‘colloid” is used by chemists for gelatinous silicate, and by Dr. Hinde 
and others for the opaline or amorphous silicate. I think that it should be limited to the former use, 
and the term ‘‘opaline” used when reference is made to the spicule of sponges and to the siliceous 
shells of diatoms, etc. 
2 Loe. cit., p. 425. 
* Cat. Fossil Sponges, 1883, Pl. XTV. 
4 As figured by Beecher; Mem. Peabody Museum of Yale Univerity, Vol. II, Part I, 1889, Pls. I-III. 
