334 LHE CEPHALOPODA 
the aperture, instead of being concave as in the Nautiloidea; finally, by 
the sutures (intersections of the septa with the shell), which form a more 
or less complex sinuous line, instead of being simple as in the 
Nautiloidea. 
Although the Ammonitoidea have a globular initial chamber which is 
absent in Nautiloidea (though it may possibly be present but caducous or 
not calcified in the latter group), their shell has the same structure as that 
of Nautilus, and was indisputably external. The chamber containing the 
body of the animal is very deep, more so than in Nautilus. Like the 
Nautiloidea, the Ammonitoidea did not possess an ink-sac. A consider- 
able number of Ammonitoidea resemble such Nautiloid forms as 
Gomphoceras, Phragmoceras, etc., in having a contracted aperture, indicat- 
ing an analogous structure of the cephalopedal organs: such contracted 
apertures may be seen in Arcestes, Lobites, Stolicekaia, and especially in 
Stephanoceras, but this contraction is carried to an extreme in Morphoceras 
pseudoanceps (Fig. 294). In this genus the aperture is almost completely 
closed by the lobes which form its borders and circumscribe five small 
orifices ; there is a central oblong orifice which probably corresponded to 
the mouth ; two circular lateral orifices, one on either side of the central, 
may perhaps have served as windows for the eyes; and the two remaining 
orifices, which are partly limited by the preceding whorl, probably served 
for the passage of the pedal appendages, and do not correspond to any 
part of the contracted apertures of Nautiloidea. A calcified structure, 
consisting of a single piece (Anaptychus) or of two symmetrical moieties 
(Aptychus) is sometimes found in the terminal chamber of Ammonitoidea : 
its constant position shows that it could not have been an operculum, and 
it is supposed to have been a calcified cartilage situated at the base of the 
funnel. 
The Ammonitoidea are, geologically speaking, younger than the rest 
of the Tetrabranchia. They appeared in the Devonian and became com- 
pletely extinct at the end of the Secondary period. They were littoral in 
habit, and lived in troops like Nautilus. Some of the coiled forms are as 
much as seventy centimetres in diameter. More than 5000 species have 
been described, and it has been found necessary to divide the originally 
single genus Ammonites first into genera, then into families, and even into 
tribes. 
TRIBE 1. RETROSIPHONATA. 
The siphuncular necks project behind the septa as in the Nautiloidea, 
These are the most ancient Ammonitoidea, belonging exclusively to the 
superior Palaeozoic strata, from the Devonian upwards. The sutures of 
the septa form simple undulations, those which point backwards being 
known as “lobes,” and those which point forward towards the aperture 
as “saddles.” 
Famity 1. Gontatiripar. Shell nautiloid with simple sutures and a 
ventral siphuncle. Genera—Goniatites, de Haan ; Devonian and Carbon- 
iferous. Anwrcestes, Mojsisovics; Devonian. Faminty 2. CLYMENIIDAE. 
Shell nautiloid; sutures simple; the siphuncle dorsal, that is to say, 
internal. Genus—Clymenia, Minster ; from the Upper Devonian. 
