20 NORTH AMERICAN INDEX FOSSILS. 



camerated structure, i. e., the greater part of the shell is divided by 

 septa into air chambers or cximercu. The last formed septum con- 

 stitutes the base of the body cr/zam^^rjust as each preceding septum 

 constituted the base of the body chamber at an earlier period when 

 the animal was younger and the shell shorter. It thus appears 

 that there is a periodic withdrawal of the animal from the base 

 of the body chamber, followed by the building of a new septum, 

 which cuts off the space which has become too small for the animal. 

 The septa are pierced by the siphonal tube or siphuncle, each per- 

 foration being bordered by a backward prolongation or tube, the 

 siphonal funnel (retrosiphonate), or a forward-bending siphonal 

 collar (prosiphonate), or both. The retrosiphonate condition is 

 characteristic of Nautiloids in general, and the prosiphonate of 

 most Ammonoids. The funnels are either tubular or nummu- 

 loidal, i. e., swollen out between the septa. Additional deposits 

 may modify them in the latter case (Actinoceras) (Fig. 1351). 

 In some Nautiloids (Holochoanites) the funnels continue back- 

 wards to the preceding septum (Fig. 1239), or even beyond, thus 

 becoming inserted in the funnel of the preceding septum (see 

 Vaginoceras, Fig. 1237), and sometimes an additional inner or 

 tubular lining (endosipholining) is present (Cameroceras). An 

 endosiphuncular filling in the form of cone-in-cone funnels (endo- 

 siphosheaths) is present in forms with a large siphuncle, and con- 

 stitutes the base of the open siphuncle formed by the last of these 

 sheaths. This is regularly conical {endosiphocone) , or compressed 

 (endosiphocoleon), while a narrow tube, the endosiphotube or 

 endosiphiincle, continues to the apex of the shell (Figs. 1235, b', 

 1236, 1239). In some primitive forms the siphuncle with its endo- 

 siphonal filling alone exists in the early stages, the camerated por- 

 tion appearing later. This '' preseptal sipho " may be alone pre- 

 served and in some cases it is marked by a pronounced contraction 

 where the cameras begin (see Nanno, Fig. 1242). Not infre- 

 quently a part of the camerated portion is destroyed in fossiliza- 

 tion, the solid siphuncle alone remaining (see Fig. 1240). 



The position of the siphuncle varies in Nautiloids from centren 

 to subventran or subdorsan, with intermediate stages as shown in 

 the subjoined diagram (Fig. 1231). In Ammonoids it is subven- 

 tran (exogastric), or in one group subdorsan (endogastric). In 

 some primitive Goniatites it may be more nearly centren. 



