110 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 1, 



Fig. 3 is from a specimen of the smaller extremity of an Orthoce- 

 ras which has been crushed, and broken along the centre, and where 

 subsequent action has caused a slight accumulation of shaly matter 

 upon the shell, showing the incipient stage of these sac-like concre- 

 tions. The surface is striated after the peculiar manner of these con- 

 cretions, and a few of the septa are shown in the middle of the lower 

 part of the figure. 



Fig. 4 is from a specimen where the original form of the shell of 

 the Orthoceras is lost, and the accumulation of the surrounding ma- 

 terial has assumed the aspect of a fleshy sac surrounding the tube as 

 the external fleshy body of the Orthoceras is supposed to envelope 

 the internal shell. 



These peculiar results of concretionary action are seen in the 

 greatest perfection in the soft, fine shales which are formed from a 

 sediment of impalpable mud, which must have been deposited in the 

 most quiet waters, as evidenced both from its character and from the 

 perfect preservation of almost all the imbedded fossils. The perfect 

 preservation of the fossils in such localities is used as an argument 

 to convince us that these soft parts may be preserved ; but we find 

 similar fossils equally well preserved in calcareous deposits, while 

 there is never any evidence of the existence of the softer parts. 

 Again, in these examples the shell is often partially or entirely de- 

 stroyed by the action of acids, apparently resulting from the decom- 

 position of iron pyrites, leaving only a cast of the interior. In the 

 calcareous deposits of the Trenton limestone, the Orthocerata pre- 

 serve their delicate shell in great perfection ; and yet among the 

 multitudes thus preserved, we find no appearance of the preservation 

 of the softer parts of the animal. 



Mr. Anthony, in a letter of January 1848, called my attention to 

 the paper cited above, and suggests that this discovery, which seems 

 so well sustained, both by evidence and high opinion, conflicts with 

 the \dews I have expressed in my * Report on the Palaeontology of 

 New York,' vol. i. I have therefore briefly examined the facts in 

 relation to this specimen, and the conclusions drawn therefrom ; and 

 I must leave it to naturalists to decide how far these conclusions can 

 be sustained, or whether they are in any degree impaired by the ac- 

 companying facts and illustrations. 



With regard to the opinions on the Orthocerata advanced in the 

 work just cited, I am far from being anxious to sustain them at the 

 expense of truth, or by the concealment or abridgement of any fact 

 connected with the subject. I have there expressed, however imper- 

 fectly, the results of my observations ; and I have even hesitated to 

 insist upon conclusions which the facts seemed to warrant. While 

 preparing that work, I examined all the authorities on this subject 

 within my reach. I found little to assist me in regard to the peculiar 

 forms and arrangement of parts of those species peculiar to our Lower 

 Silurian rocks, and was forced therefore to depend on my own inves- 

 tigations. I have proposed a generic name (F?idoceras), indicating 

 a peculiarity in the mode of development and growth, simply ; while 

 I believe the facts would justify a still wider separation than that of 



