3io 



MANUAL OF ZOOLOGY. 



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

 shells). 



In the family Ammonitida (fig. 112), 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). 



,0 ' Q .0 



Fig. 112. 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 Baculite', bb Ceratite', cc Goniatite\ dd Clymenia', e e Nau- 

 tilus 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 Nautilida 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. 113). 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 ex- 

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

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



