Asymmetrical, inequivalve, fixed, with extensive pallial sutures; no
siphons. Two adductors. Foot reduced and without byssus. Shell thick,
without pallial sinus.
Fam. 1. _Chamidae._--Shell with sub-equal valves and prominent
umbones more or less spirally coiled; ligament external. _Chama.
Diceras_; Jurassic. _Requienia_; Cretaceous. _Matheronia_;
Cretaceous.
Fam. 2. _Caprinidae._--Shell inequivalve; fixed valve spiral or
conical; free valve coiled or spiral; Cretaceous. _Caprina._
_Caprotina. Caprinula_, &c.
Fam. 3. _Monopleuridae._--Shell very inequivalve; fixed valve
conical or spiral; free valve operculiform; Cretaceous.
_Monopleuron. Baylea._ The two following families, together known
as Rudistae, are closely allied to the preceding; they are extinct
marine forms from Secondary deposits. They were fixed by the conical
elongated right valve; the free left valve is not spiral, and is
furnished with prominent apophyses to which the adductors were
attached.
Fam. 4. _Radiolitidae._--Shell conical or biconvex, without canals
in the external layer. _Radiolites. Biradiolites._
Fam. 5. _Hippuritidae._--Fixed valve long, cylindro-conical, with
three longitudinal furrows which correspond internally to two
pillars for support of the siphons. _Hippurites. Arnaudia._