THE CEPHALOPODA. 
177 
The characters are entirely derived from the structure of the shell. With the exception 
of the two living species of the genus Nautilus all the others are extinct. It vi^ill, there- 
fore, be necessary to review these genera in detail in order to understand their natural 
relations and true affinities with the other members of the sub-order to which they belong. 
Suh-order A.—]^km\WYQk., Wrif/hf, l^^Q. 
Shell with a large body-chamber, simple aperture. The siphuncle for the most part 
central or columellar, with the tube directed backwards. Septa simple and concave 
towards the aperture ; on the removal of the shell the margin of the septa shows simple 
curved lines, corresponding with the curve of the septum (fig. 27). 
The following groups belonging to this order have been described : 
1. Family Nautilid^e, Owen, 1836. — Shell always convolute and discoidal. 
Genus. — Nautilus, Breynius, 1732. Shell camerated, spiral, discoidal; whorls 
regularly convoluted on same plane, chambers separated by concave septa; siphuncle 
central or subcentral ; Silurian to present period. 
2. Family LixuiTiDiE, Wriyht, 1880. — Shell convolute and discoidal in youth, and in 
old age projecting forwards in a straight line. 
Genus. — Lituites, Breynius, 1732. Shell camerated, spiral, discoidal ; early whorls 
distinct and contiguous, but the last ceases to be contiguous, and projects forward in a 
straight line, so as to form, with the first, a kind of crooked staff. The last chamber very 
large, with a simple circular opening, destined to contain the animal. Siphuncle central or 
sub-central. Silurian. 
Genus. — PIortolus, Montfort, 1808. Shell camerated, spiral, discoidal, with whorls 
non-contiguous. The last projects in a straight line ; and the whole forms an elongated 
crooked staff. Silurian. 
3. Family Aploceratidjs, Wriyht, 1880. — Shells more or less bent. 
Genus. — Aploceras, d' Orhig., 1847 ; Corniculina, Miinster, 1843. Shell resembles 
a bent horn, regularly arched. Siphuncle subcentral. Carboniferous Limestone. 
4. Family ORTi-iocERATiDiB, Woodimrd, 1851. — Shells straight. 
Genus. — Orthoceras, Breynius, 1732. Orthoceratites, Auct. ; Conilites, 
Lamarck, 1822; liioLiTHEs, Eichwald, 1840; Cycloceras, M'Coy, 1844. Shell 
