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MANUAL OF ZOOLOGY. 



or less completely plain ; and the ' siphuncle ' is central, 

 subcentral, or internal (i.e. on the concave side of the curved 

 shells). 



In the family Ammonitidce (fig. 86), 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). 



d 



Fig. 86. Diagram to illustrate the position of the siphuncle and the form of the 

 septa in various Tetrabranchiate Cephalopoda. The upper row of figures 

 represents transverse sections of the shells, the lower row represents the 

 edges of the septa, a a. Ammonite or Daculite; bb. Ceratite; cc. Goniatite ; 

 d d. Clymenia ; e e. Nautilus or Orthoceras. 



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 : 



Nautilida. 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 

 Nautilidoe is the Orthoceras. In structure this was doubtless 

 essentially identical with the Nautilus, but the shell, instead of 

 being coiled into a spiral lying in one plane, was extended in 

 a straight, or nearly straight, line. Orthoceratites of more 

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



