Family. Trimerellidae
Shell thick, calcareous, inequivalve; beak of ventral valve usually prominent; rudimentary teeth maybe present; hinge area well developed, with pseudo-deltidium. In interior of valves muscular platform, “crescent,” and sometimes sub-umbonal chambers (see p. 494, Fig. [323]).
Range.—Ordovician and Silurian; maximum in Wenlock.
Principal Genera.—Trimerella, Monomerella, Dinobolus, Rhinobolus.
II. Testicardines
Family. Productidae
Shell entirely free, or fixed by ventral valve or spines. Concavo-convex, more or less covered with tubular spines. Hinge line straight. Hinge-teeth absent or rudimentary.
Cardinal process prominent.
Reniform impressions in dorsal valve.
(For muscular impressions see p. 501, Fig. [333].)