356 MANUAL OF ZOOLOGY. 



In the family Nautilida (fig. 149), the " septa " of the shell 

 are simple, curved, or slightly lobed ; the " sutures " are more 

 or less completely plain ; and the " siphuncle J; is central, 

 sub-central, or internal (i.e., on the concave side of the curved 

 shells). 



In the family Ammonitidce (fig. 149), on the other hand, the 

 septa are folded and complex ; the sutures are angulated, zig- 

 zag, lobed, or foliaceous; and the siphuncle is external (i.e., on 

 the convex side of the curved shells). 



In both these great types of shell, a series of representative 

 forms exists, resembling each other in the manner in which the 

 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 Nautilidce. or of the Ammonitidce. The following table 

 (after Woodward) exhibits the representative forms in the two 

 families : 



Nautilidfs. Ammonitidtz. 



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. 150). In structure this was 



Fig. 150. Orthoceras exploratory 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 



