434 MANUAL OF ZOOLOGY. 



shell is folded or coiled, but differing in their fundamental 

 structure. All these different forms may be looked upon as 

 produced by the modification of a greatly elongated cone, the 

 structure of which may be in conformity with the type either 

 of the Nautilidcz or of the Ammonitidce. The following table 

 (after Woodward) exhibits the representative forms in the two 

 families : 



Nautilidce. Ammonitidce. 



Shell straight, Orthoceras, . Baculites. 



bent on itself, . . . Ascoceras, . . Ptychoceras. 



curved, Cyrtoceras, . Toxoceras. 



spiral, Trochoceras, . Turrilites. 



discoidal, Gyroceras, . . Crioceras. 



discoidal and produced, Lituites, . . . Ancyloceras. 



involute, Nautilus, . . Ammonites. 



After the Nautilus itself, the most important form of the 

 Nautilidce is the Orthoceras (fig. 235). In structure this was 



Fig. 235. Orthoceras explorator, Billings, i. Side view of a fragment, showing 

 the septa. 2. Transverse section of the same, showing (s) the siphuncle. 



doubtless essentially identical with the Nautilus, but the shell, 

 instead of being coiled into a spiral lying in one plane, was ex- 

 tended in a straight, or nearly straight, line. Orthoceratites of 

 more than six feet in length have been discovered, but in all, 

 the body-chamber, in which the animal was lodged, appears to 

 have been comparatively small. The siphuncle is usually very 

 complex in structure, and was calcareous throughout its entire 

 length. 



The structure of the shell in the Ammonitidce is exactly that 

 of the Pearly Nautilus, consisting of an outer porcellanous 

 and an inner nacreous layer. The body-chamber was rather 

 elongated than laterally expanded or dilated. The simplest 

 form of the Ammonitidce is the Baculite, in which the shell is 

 straight, like that of an Orthoceras, while the septa have the 

 characters of those of an Ammonite, and the siphuncle is ex- 

 ternal. In the Turrilite (fig. 237) the structure of the shell is 

 the same, but it is coiled into a turreted spiral. In the Am- 



