50 NAUTILIDA—AMMONITID. 
not have been like the Spirula, an internal shell, but must have 
been closely 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. 
In 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 hectocotylized, having lost 
all trace of acetabular organization. 
Orpver II. TETRABRANCHIATA. 
Famity NAUTILID A. 
-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. 
Faminry AMMONITID &. 
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 Goniatites, 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. 
