FOSSIL CEPHALOPODS. 305 



creature. Each chamber was formerly filled by the 

 animal, which retired from it as it grew larger, and 

 formed another with more room ; but it maintained 

 its organic connection with all of these deserted 

 chambers by means of the siphuncle. 



These shelled Cepha- 

 lapods ( Tetr -abranchiate or 

 " four-gilled ") are sepa- 

 rated in the Nautilus and 

 Ammonite families. In 

 the former, the divisions 

 (septa) of the chambers 

 are simple and curved, and 

 the edges (sutures) plain, 

 whilst the siphuncle usu- 

 ally runs through the 

 middle (central), as above Fig " 



described. In a few instances, however, the siphuncle 

 is ventral, or at the base of the chambers. 



In the Ammonites, on the contrary, the septa are 

 folded and very complex, and the sutures are zig- 

 zagged, foliated, or irregularly lobed ; the siphuncle 

 or air-tube is on the outside of the chamber (dorsal). 



These are very well-marked and easily recognized 

 points of difference. It is as well to remember them, 

 for, as far as external appearance goes, it is singular (as 

 Dr. Nicholson has shown) how one group mimics the 

 other, the real fact being that they were built up on 

 the same external architectural lines. Thus, in the 



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