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chambered straight, spiral, curved, or discoidal ; aperture 
guarded by processes, or closed by an operculum ; the septa 
giving rise to sutural markings, which may be angulated or 
complexly foliated ; provided with an external siphuncle. 
All the species fossil, includes genera Goniatites, Bactrites, 
Ceratites, Ammonites, Criocei^, Toxoceras, Ancyloceras> 
Scaphites, Turrilites, Hamites, Ptychoceras, Baculites, &;c. 
Family B. JSfautilidcB. — The Nautilus being the only 
known living type, the diagnosis of the order serves for this 
family also. Shell : curved, discoid or straight ; sutural 
markings simple or contracted, with central or sub- central 
siphuncle. 
Genera. — Nautilus, Aturia, Lituites, Trochoceras, Ortho- 
ceras, Gomphoceras, Phragmoceras, Cyrtoceras, Gyroceras, &c. 
Order II., Dibraxchiata. — Animal without true external 
shell, sometimes with a rudimentary internal one ; has sessile 
eyes, horny mandibles, two gills, not more than ten elongate 
arms, which are provided with suckers ; a funnel which forms 
a complete tube, and an ink-bag. 
Section (1), Decapoda. — Animal having eight arms, and 
two clavate tentacles (ten altogether), bearing pedunculated 
suckers, strengthened by a horny ring ; having eyes movable, 
body elongate, and furnished with a pair of fins, and an 
internal shell, gills partly attached. 
Family C. Spirulidce. — Animal provided with an internal, 
pearly shell, having a central siphuncle, and being discoidal, 
with the whorls separated. 
Genus. — Spirula. 
Family D. Sepiadce. — Animal furnished with an internal 
calcareous broad plate (cuttle-bone) which terminates behind 
in an imperfect chamber. Genera: Sepia Spirulirostra, Belop- 
tera, &c. 
Family E. Bekmnitidce. — All fossil; shell internal, consist- 
ing of a pen (gladius), a chambered portion (phragmacone), 
