cites_, Carboniferous.

Fam. 11. _Bactritidae_. Shell straight, conical, siphuncle narrow
and marginal, necks long, infundibuliform, sutures undulating.
_Bactrites_, Silurian and Devonian.

_Sub-order 2. Ammonitoidea_,--Initial chamber spheroidal; siphuncle
narrow and simple; septa convex towards aperture; sutures complex.

_Tribe 1. Retrosiphonata_.--Siphuncular necks projecting behind the
septa as in Nautiloidea. Sutures form simple undulations. Occur
exclusively in Palaeozoic strata from Devonian upwards.

Fam. 1. _Goniatitidae_. Shell nautiloid, with simple sutures and
ventral siphuncle. _Goniatites_, Devonian and Carboniferous.
_Anarcestes_, Devonian.

Fam. 2. _Clymeniidae_. Shell nautiloid, with simple sutures,
siphuncle dorsal, that is, internal. _Clymenia_, Upper Devonian.

_Tribe 2. Prosiphonata._--Siphuncular necks projecting in front of the
septa. Sutures form deeply indented lobes and saddles.

Fam. 1. _Arcestidae_. Globular and smooth or nearly smooth, with
reduced umbilicus, terminal chamber very deep, an aptychus present.
_Popanoceras_, Permian. _Cyclolobus_, Permian, _Arcestes_, Trias.
_Lobites_, Trias.

Fam. 2. _Tropitidae_. Shells globular, but having radiating and
tuberculated costae. _Thalassoceras_, Permian. _Tropites_, Trias.
_Sibirites_, Trias.

Fam. 3. _Ceratitidae_. Shells coiled, with a large umbilicus,
terminal chamber short, sutures with simple saddles. _Trachyceras_,
Upper Trias. _Ceratites_, Trias. _Dinarites_, Trias.

Some genera with helicoidal shells are related to these coiled forms,
viz. _Cochloceras_, Trias; also some straight forms, e.g.
_Rhab-doccras_, Trias.

Fam. 4. _Pinacoceratidae_. Shell compressed, smooth, terminal
chamber short, sutures very complicated, convex. _Pinacoceras_,
Trias.

Fam. 5. _Phylloceratidae_. Shell coiled, the whorls overlapping each
other, sutures formed of numerous lobes and saddles. _Phytloceras_,
Jurassic.

Fam. 6. _Lytoceratidae_. Shell discoid, whorls loosely united or
uncoiled, sutures deeply indented, but with only three saddles and
lobes. _Lytoceras_, Jurassic and Cretaceous. _Macroscaphites_,
Cretaceous. _Reunites_, Cretaceous. _Ptychoeeras_, Cretaceous.
_Turrilites_, Cretaceous. _Baculites_, Cretaceous.

Fam. 7. _Ammonitidae_. Shell coiled, with narrow whorls which do not
embrace one ano