204 THE LIAS AMMONITES. 



ANALYSIS OE THE FAMILIES AND GENERA OE THE FOSSIL 

 CEPHALOPODA.^ 



First Ordek. — 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, 

 Argonautidae (Reeve, 1841). One fossil genus, Argonauta. 



L Argonauta, Linne, 1758. 

 Tertiary. 



Section II. — Decapoda. 



II. — Shell lance-shape, slender, with central ridge, generally terminating in a slight 

 expansion of no great length without air chambers. Family, ONYCHOTEUTHiDiE 

 (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 sinuous, 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. 



' Drawn up by tlie Rev. T. "WiltsLire, M.A., F.G.S., in 1867, see page 203. 



