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 will, 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. 



>5'«3-or^e/-^.— NAUTILOID A, rn>/^^, 1880. 



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 Hnes, 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, Brepiius, 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 LiTUiTiDiE, Wright, 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. — Hortolus, 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 Aploceratid^, Wright, 1880. — Shells more or less bent. 



Genus. — Aploceras, cV Orbig., 1847 ; Corniculina, Milnster, 1843. Shell resembles 

 a bent horn, regularly arched. Siphuncle subcentral. Carboniferous Limestone. 



4. Family Orthoceratid^, Woodward, 1851. — Shells straight. 



Genus. — Orthoceras, Breynius, 1732. Orthoceratites, Auct. ; Conilites, 

 Lamarck, 1822; Hyolithes, Eichwald, 1840; Cycloceras, M'Coy, 1844. Shell 



