39 



1. Shell spiral, the turns touching, in the same plane. 



57. Ceratites, De Haan, 1825 From Triassic into Cretaceous. 



2. Shell straight. 



58. Baculina, D'Orbigny, 1850 Cretaceous. 



XII. — Shell many chambered ; the last (exterior) chamber large, siphuncle 

 marginal (dorsal) ; the edges of the partitions (septa), which 

 separate the chambers, foliaceous. Ammonitid^e (Owen, 1838). 

 Thirteen genera : Ammonites, Crioceras, Toxoceras, Scaphites, 

 Ancyloceras, Ptychoceras, Hamites, Hamulina, Baculites, Tur- 

 rilites, Helicoceras, Heteroceras, and Anisoceras. 



1. Shell spiral, in the same plane. 



59. Ammonites, Bruguiere, 1789 Turns of the shell in contact, overlapping ; 



= Ammonis cornu, Auctorutn. siphuncle on the exterior side ; Triassic 



= Planorbites, Lamarck, 1799. into Cretaceous. 



= Planulites, Lamarck, 1801. 



= Amaltheus, Montfort, 1808. 



= Orbulites „ „ 



= Grlobites, De Haan, 1825. 



= Planites, ,, 



Aptychus, H. von Meyer, 1831 ... Opercula, or doors, closing the external 

 = Trigonellites, Parkinson, 1811. opening of the Ammonite shell. 



= Tellinites, Schlotheim, 1813. 

 = Miinsteria, Deslongchamps, 1835. 

 = Ichthyosiagones, Bourdet, 1822. 



Peltarion, Deslongchamps, 1859 ... Circular, or somewhat oval calcareous 



plates, with an undulating margin, sup- 

 posed to be connected with the mandibles 

 of the Ammonite animal ; Lias into 

 Oolite.) 



2. Shell spiral, turns of the shell not in contact, in the same plane. 



60. Crioceras Cretaceous. 



= Crioceratites, Leveille, 1837. 

 = Tropa3um, Sowerby, 1840. 



3. Shell an arc of a spiral, in the same plane. 



61. Toxoceras, D'Orbigny, 1840 Cretaceous. 



= Toxocerus, King, 1844. 



4. Shell changing from a spiral, in the same plane. 



62. Scaphites, Parkinson, 1811 Turns of the shell at first spiral, in con- 



tact, and overlapping, afterwards tan- 

 gentially divergent ; finally recurved 

 and not in contact ; Cretaceous. 



63. Ancyloceras, D'Orbigny, 1840 ... Turns of the shell at first spiral, not in 



in contact, afterwards tangentially di- 

 vergent ; finally recurved, and not in 

 contact ; from Oolitic into Cretaceous 

 formations. 



5. Shell forming straight and curved lines, in the same plane. 



64. Ptychoceras, D'Orbigny, 1840 ... The shell at first straight, afterwards 



= Ptychocerus, King, 1844. curved, and next straight, the turns in 



contact ; Cretaceous. 



