LowEE Heldeebeeg Fauna. 



281 



the intermural cavity with the surrounding medium. Kauff 

 denies the existence of both pores and canals, and I have been 

 unable to find anything analogous in the Lower Helderberg 

 specimens. 



V. The Chemical Constitution. 

 The conclusions to be deduced from the appended table confirm 

 those of Rauff with regard to the chemical composition of 

 Reoeptacdlites, at least so far as to show that it was originally 

 calcareous and not siliceous, as claimed by Hinde. Specimens of 

 Reoeptaculites from the Helderberg mountain agree with the 

 Cephalopods in their mode of preservation, and it seems probable 

 that, like them, Receptacolites had a test composed of aragonite. 



Natural 



Etching 



Acidaspls shale 



Soft shale 



Limestone 



Bryozoa. 



Brachlopods. 



Hindia. 



Calcareous . . 

 Siliceous — 

 Calcareous . . 

 Calcareous . . 

 Siliceous 



Calcar 



Silic 



Silic 



Pyrite 



Semisilic .... 



Calcar 



Silic . 



Calcar 



Calcar 



Silio 





Receptaculltes. 



Casts (colored) . . 



Pyrite 



Ferrug. Impress 

 Casts (colored).. 



Cephalopod 



Aragonite. 



Casts. 

 Pyrito. 

 Pyrite. 

 Casts. 



In considering the various features belonging to this species as 

 stated above, the view that the examples of REOEPTAcuLirES 

 which have been regarded as representing the true structure of 

 the organism, and studied as such, are probably only casts or 

 infiltrations, must have suggested itself. From many considera- 

 tions, this seems the inevitable conclusion, yet, if the Lower 

 Helderberg specimen is Receptac ulites, it is not a cast, but repre- 

 sents the original organism more truthfully than any example 

 yet described. 



In some respects the structures referred by different writers to 

 Reoeptaculites are so analogous to those seen in this specimen, 

 that it seems more than probable that they are casts ; yet certain 

 features, as described, are impossible to explain on that basis, 

 and the hypothesis must be, in many oases, considerably modi- 

 fied. Much, however, can be explained by the mode of preser- 

 vation. The processes of replacement and of fossilization are 

 known only as results. E'o one has been able to study them in 

 operation. To make a priori assertions in regard to them is 

 impossible, since conditions beyond consideration might at any 

 moment change what would seem the probable course. This is 

 36 



