Shell external structure
Beak - pointed extremity (front) of shell that marks the beginning of shell growth
Chilidium - plate extending across the notothyrium, partly closing it
Delthyrium - notch beneath beak of pedicle valve for passage of pedicle, extends to the
Interarea - generally flat surface between beak and posterior margin of either valve
Notothyrium - triangular opening at middle of interarea of the brachial valve, furnishing an opening (with deltyrium) for the pedicle