CEPHALOPODA. 



193 



known, except that it must certainly serve to maintain the 

 vitality of the shell. 



FIG. 91. The Pearly Nautilus (Nautilus pompitius). a Mantle ; & Its dorsal fold ; c Hood; 

 o Eye ; t Tentacles ; / Funnel. 



Of the fossil Tetrabranchiata the most important are the 

 Orthocerata and the Ammonites. The Orthocerata (Fig. 92) 

 played a very important part in the seas of the Palaeozoic or 

 Ancient-life period of the earth's history, in which they ap- 

 parently filled the place now taken by the predacious cuttle- 



FIG. 92.Orthocera8 arplorator. 1. Side view of a fragment, showing the edges of the 

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



fishes. They agreed with the Nautilus in having a many- 

 chambered shell, divided by curved partitions, perforated by a 

 tube or siphuncle. The shell, however, differed from that of 

 the Nautilus in not being curved or coiled up, but in being 



