204 
THE LIAS AMMONITES. 
ANALYSIS OE THE FAMILIES AND GENERA OE THE FOSSIL 
CEPHALOPODA.^ 
First Order. — DIBRANCHIATA, containing Section I. Octopoda, and 
Section II. Decapoda. 
Section I. — Octopoda. 
I, — Shell involute, the turns in the same plane, thin, one chambered. Family, 
Argonautidse (Reeve, 1841). One fossil genus, Argonauta. 
1. Argonauta, Linne, 1758. 
Tertiary. 
Section //.—Decapoda. 
II. — Shell lance-shape, slender, with central ridge, generally terminating in a slight 
expansion of no great length without air chambers. Family, Onychoteuthid^ 
(Gray, 1847). Three fossil genera, Enoploteuthis, Plesioteuthis, and Ommas- 
trephes. 
2. Enoploteuthis, D'Orbigny, 1840. 
Sides sinuous, with lateral expansions ; central ridge keeled ; Oolitic. 
3. Plesioteuthis, Wagner, 1860. 
Sides sinuouSj with central and two side ridges ; projection arrow-shaped ; arms with 
hooks ; Liassic. 
4. Ommastrephes, D'Orbigny, 1840. 
Sides generally straight, contracted towards base, point expanded, central and side 
ridges ; Oolitic. 
1 Drawn up by the Rev. T. Wiltshire, M.A., F.G.S., in 1867, see page 203, 
