TKTRAHKANOUIATA. 



207 



presenl accented. The following .s\iioi)sis will exhibit these 

 variations in the <>enera. 



Ammonitid^. 



Order IT. TETRABRANCHIATA. 



F'ainily 1. Nautilida\ Septa simply cuirved, concave on the 

 outer face, sutures simple, or undulate or lobed ; mouth simple ; 

 siplional opening nearly central. Shell l)ut little scul])tured, or 

 smooth. 



Six living and about fiOO fossil species. 



Family 2. AmmonitidtE. Septa convex in their median sec- 

 tion, sntiires complex, lobed, ramified or denticulated ; septal 

 tnbe cylindrical and always directed forwards ; siphuncle cylin- 



