284 Report of the State Geologist. 



Most writers on Eeceptactjlites have reached different eon- 

 elusions as to its original composition. Hinde believes that it 

 was siliceous ; Rauff that it was calcareous ; Gdmbel that it was 

 aragonite, and Billings that it was calcite or in part coriaceous. 

 These variations in results may be explained by the fact that infil- 

 tration products only have been investigated, the nature of the 

 material being that which at the time was most plentifully 

 dissolved in the sea-water of a given locality. This notion 

 naturally modifies conclusions reached under such conditions- 

 However, my own conclusions agree either with those of Gumbel 

 that Receptaculites was originally composed of aragonite, or of 

 Billings that it was chitinous. The matrix of the pyritized 

 specimen is shaly limestone. The radial tubes and the space 

 between them are filled by calcite or (?) celestite. This feature 

 is in marked contrast with ordinary specimens, for, as Billings 

 says, " the space between the tubes is almost always filled with 

 the rock of the same kind as that in which'the fossil is imbedded.'^ 



The majority of examples studied by Hinde, Billings, and 

 others, are probably like H. Owen% described below, where the 

 cavity left by the dissolution of the original organism has been 

 filled by a structureless deposition from infiltering waters. It 

 seems as if the more unique details of structure may have been 

 derived from an infiltration, filling the organism itself, or from 

 others where a maceration of the skeleton has left upon the 

 cast traces which usually would not be received. Replace- 

 ments by silica or calcite may also in some instances have been 

 investigated. 



The conclusions of the above writers are of a general character, 

 and it has been possible to discuss them only in a general way. 

 However, specimens of both B. Nejptuni and R. Oweni have 

 been examined and afford a definite basis for comparison. 



Receptaculites Oweni. 

 This species commonly occurs as tabular fragments of the 

 basal portion of the organism. Specimens from Illinois usually 

 have a thickness of 5 mm. near the basal pole, increasing distally 

 to 20 mm. or more, and sometimes attain a very large size. The 

 material is a granular dolomite, having much the appearance of 

 yellowish sandstone. The outer surface is marked off into . 



