50 NAUTILID^ — AMMONITID^. 



not have been like the Spirula, an internal shell, but must have 

 been closel}^ related to Nautilus.* 



According to some recent investigators, there is a marked 

 resemblance between the recent Spirula and the fossil Ammonites, 

 particularly in the initial whorl, and a difference in the latter 

 character between Ammonites and Nautilus which is thought to 

 indicate that the Ammonites should be separated from the tetra- 

 branchiate and united with the dibranchiate cephalopods; If 

 this should prove to be so, then ,the Spirula will assume a new 

 importance to us as the last vestige of a numerous group, else 

 extinct. 



til Proc. Zool. Soc.^ London, 1880, Prof Owen describes and 

 figures a male Spirula. The ventral pair of arms are modified 

 for the sexual purpose, but are not hectoeotylized, having lost 

 all trace of acetabular organization. 



Order II. TETRABRANCHIATA. 

 Family NAUTILIDJE. 



Septa simply curved, concave on the outer face, sutures 

 simple, or undulate or lobed ; mouth simple ; siphonal opening 

 nearly central. Shell but little sculptured, or smooth. 



Six living and over 2000 fossil species. 



Family AMMONITIDiE. 



Septa convex in their median section, sutures complex, lobed, 

 ramified or denticulated ; septal tube cylindrical and always 

 directed forwards ; siphuncle cylindroid, small, marginal, the 

 siphonal investment more or less solid and persistent. Fossil 

 only, several thousand species known. 



Nearly 5000 fossil species of cephalopod shells have been 

 referred to the tetrabranchiates, although it has been recently 

 suspected that at least a large portion of these were internal 

 shells like the Spirulas and referable therefore to the dibran- 

 chiata. Only a half-dozen recent species are known, all belonging 

 to the genus Nautilus. 



The tetrabranchiate shell is essentially an elongated cone, 

 divided off into chambers by partitions, and siphunculated. 

 These septa have simply curved edges in Nautilus and Ortho- 

 ceras, they are zigzag in Groniatites, or foliaceous, forming com- 

 plicated lobes in Ammonites. The shell may be straight, curved, 

 open or close spiral, and even vary in form at different ages, and 

 these variations, when well understood, will doubtless cause a 



* Owen, on the Relative Positions to their Constructors of the Cham- 

 bered Shells of Cephalopods. Zool. Proc, 955, 1878. 



