214 
THE LIAS AMMONITES. 
XL — Shell many chambered ; the last (exterior) chamber large ; siphuncle marginal 
(dorsal); the edges of the partitions (septa), which separate the chambers, 
waved, with many small circular elevations (crenulations) at the base of each 
wave. Ceratitid^. Two genera : Ceratites and Baculina. 
1. Shell spiral, the turns touching, in the same plane. 
57. Ceratites, de Haan, 1835, 
Eroni 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^ (Owen, 1836). Thirteen genera: Ammonites, 
Crioceras, Toxoceras, Scaphites, Ancyloceras, Ptychoceras, Hamites, Hamulina, 
Bacuhtes, Turrilites, Helicoceras, Heteroceras, and Anisoceras. 
1. Shell spiral, in the same plane. 
59. Ammonites, Bruguiere, 1789. = Ammonis cornu, Auctorum. = Planorbites, 
Lamarck, 1799. = Planulites, Lamarck, 1801. = Amaltheus, Montfort, 1808. 
= Orbulites, Montfort, 1808. — Globites, de Haan, 1825. — Planites, de Haan, 
1825. 
Turns of the shell in contact, overlapping ; siphuncle on the exterior side ; Triassic 
into Cretaceous. 
Aptychus, H. von Meyer, 1831. = Trigonellites, Parkinson, 181 1. = Tellinites, 
Schlotheim, 1820. = Miinsteria, Deslongchamps, 1835. = Ichthyosiagones, Bourdet, 
1822. 
Opercula, or doors, closing the external opening of the Ammonite shell. 
Peltarion, Deslongschamps, 1859. 
Circular, or somewhat oval calcareous plates, with an undulating margin, supposed to 
be connected with the mandibles of the Ammonite animal ; Lias into Oohte. 
