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. 1 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. 2 



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 

 Medusse, 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 

 medusa?. The points of resemblance to the sponges are found in making 

 comparison of the exterior form with Hallirlioa costata Lamx. 3 and Brachio- 

 spongia digitata Owen, 4 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 spicuke. 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 medusa?, 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 PI. VIII and the Lower Cam- 



1 1 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 spicuhe of sponges and to the siliceous 

 shells of diatoms, etc. 



2 Loc. cit., p. 425. 



3 Cat. Fossil Sponges, 1883, PI. XIV. 



4 As figured by Beecher ; Mem. Peabody Museum of Yale Uni verity, Vol. II, Part I, 1889, Pis. I— III. 



