122 Cephalopoda of the Cincinnati Gh'oup. 



formed, traversed by the siphon, which perforates its anterior wall or 

 septum ; by a repetition of the same process, a second chamber is 

 formed, included witliin two perforated septa ; and similar, but wider, 

 partitions continue to be added, C(mcurrently with the formation of the 

 new layers, which extend and expand the mouth of the shell, until the 

 animal acquires its full growth, which is indicated by the body havmg 

 receded, for a less distance from the penultimate septum, before the 

 formation of the last septum is begun." 



The fossil shells of this class, found in the Cincinnati Group, appear 

 to have possessed a composition and structure ditierent from that of 

 the shells of any other class. Their peculiarity is manifested by the 

 molecular changes, that seem to have taken place in all parts of the 

 shell — in the siphuucle and septa, as well as in the exterior shell. The 

 appearance presented by the specimens, which show this molecular 

 change, is that of a shell having been melted or softened and run to- 

 gether. Some specimens appear as if the side most distant from the 

 siphuncle had melted down upon it, and then the siphuncle had 

 opened longitudinally and formed a row of nodes on each side of 

 the opening. Flattened specimens of this kind have led to the 

 suggestion, on the part of some, that the shell might have been flat 

 on one side, with the siphuncle on the outside. Other specimens 

 appear as if the siphuncle and septa had melted down uiion one side, 

 and others, as if part of the exterior shell had run in upon the septa. 

 These molecular changes seem to have taken place in a high degree 

 in one part of a specimen, while another i)art remains unchanged. 

 From a knowledge of the foregoing facts, and an examination of nu- 

 merous specimens of OrtJwceras, I am led to the conclusion, that all 

 species of this genus found in our rocks are conical shells, having a 

 (urcular section. 



The shells are not porous, like those of the Brachiopoda , nor horny, 

 like the Ciuatacea ; neither do they present the same appearance, af; 

 do the LameUihranchiata, or Gasteropoda. The exact diffei-ence, how- 

 ever, must be hereafter pointed out, from chemical analysis or micro- 

 scopical examination. At present, I characterize them as distinct in 

 structure and composition, for the reason, that the shells of no other 

 class appear to have undergone these molecular changes, that present 

 the appearance of the melting and running together of different partK 

 of the shell. 



The siphuncle appears in some species of Orthoceras to be always 

 central. In other species, however, it appears to cross, from a subcen- 

 tral or marginal position on one side to the same position on the other. 

 From these facts, I infer, that there can, properly, be no dorsal or ven- 

 tral side to these shells. 



