THE CEPHALOPODA. 
207 
which has a marginal canal (siphuncle) ; the thin, shelly covering (conotheca) investing 
the chambers, prolonged on one side into a broad lobe (pro-ostracum, pen) ; animal not 
clearly known, double rows of hooks on the arms ; from Liassic into Cretaceous forma- 
tions. 
17. Belemnitella, d'Orbigny, 1840. 
Shell (guard) in its highest part, marked by a short longitudinal slit ; Cretaceous. 
18. BelexMnoteuthis, Pearce, 1842. = Kelaeno, Miinster, 1836. = Acanthoteuthis, 
Miinster, 1839. = Plesioteuthis, Wagner, 1860. 
Shell consisting of a chambered cone, having at its upper part a horny pen with thin 
side bands, and at its lowest part a thin fibrous guard, with two diverging ridges ; 
animal with arms and horny hooks, fins large ; Oolitic. 
19. CoNOTEUTHis, d'Orbigny, 1840. 
Chambered cone slightly curved, pen elongated and very narrow ; Cretaceous. 
20. XiPHOTETJTHis, Huxley, 1864. 
Shell cylindrical ; chambered cone long and narrow ; pen very long and slender, sec- 
tion an ellipse ; Liassic. 
Second Order.— TETRABRANCHIATA. 
VIII. — Shell many chambered; the last (exterior) chamber large, siphuncle generally 
subcentral ; the edges of the partitions (septa) which separate the chambers 
either without folds, or slightly undulating. Family, Nautilid^. Thirty 
fossil genera : Nautilus, Discoceras, Ophidioceras, Gyroceras, Cyrtoceras, 
Cyrtocerina, Oncoceras, Streptoceras, Piloceras, Lituites, Lituunculus, Ortho- 
ceras, Tretoceras, Huronia, Actinoceras, Ormoceras, Aulacoceras, Endoceras, 
Cameroceras, Trochoceras, Hercoceras, Gomphoceras, Phragmoceras, Ascoceras, 
GlossoceraSj Aphragmites, Aturia, Nothoceras, Bathmoceras, and Gonioceras. 
21. 
Division A. — Edges of septa without folds. 
1. Shell spiral, in the same plane. 
Nautilus, Belon, 1553. = Nautilus, Breynius, 1732. 
= Planorbites, 
