The young Brachiopod in all its species is protected by an embryonic shell called the " protegulum," which sometimes persists in the umbones of the adult shells but is more usually worn off.
A, b, right and left valves of the shell; c, d, the umbones or short arms of the lever; e, f, the long arms of the lever; g, the hinge; h, the ligament; i, the adductor muscle.
Solenopsidae.-Shell equivalve, greatly elongated, umbones very far forward.
Hinge pliodont, that is to say, it has numerous teeth on either side of the umbones and the teeth are perpendicular to the edge.
Philobryidae.-Shell thin, very inequilateral, anterior part atrophied, umbones projecting.
Trigonia; shell sub-triangular, umbones directed backwards.
Prasinidae.-Shell inequilateral, with anterior umbones and prominent anterior auricula; cardinal border arched..
Callocardiidae.-Siphons present; external gill-plate smaller than the internal; umbones not prominent.
28.-Lateral view of a umbones and internal ligament.
Chamidae.-Shell with sub-equal valves and prominent umbones more or less spirally coiled; ligament external.