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. Belemnoteuthis, 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. CoNOTEDTHis, d'Orbigny, 1S40. 



Chambered cone slightly curved, pen elongated and very narrow ; Cretaceous. 



20. XiPHOTEUTHis, 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, 

 Glossoceras^ Aphragmites, Aturia, Nothoceras, Bathmoceras, and Gonioceras. 



Division A. — Edges of septa without folds. 

 1. Shell spiral, in the same plane. 

 21. Nautilus, Belon, 1553. := Nautilus, Breynius, 1732. = Planorbites, 



