Very asymmetrical, with a single large posterior adductor. The heart
is not contained in the pericardium, lies dorsad of the rectum and
gives off a single aorta anteriorly. The reflected borders of the
inner gill-plates of either side are fused together in the middle
line. The gonads open into the kidneys and the right gonad extends
into the mantle. Shell thin; animal fixed.
Fam. 1. _Anomiidae._--Foot small; inferior (right) valve of adult
perforated to allow passage of the byssus. _Anomia_; byssus large
and calcified; British. _Placuna_; byssus atrophied in adult.
_Hypotrema_. _Carolia_. _Ephippium_. _Placunanomia_.