Umbo of ventral valve.
The ventral valve is usually the larger, and in many genera, such as Terebratula and Rhynchonella, has a prominent beak or umbo, with a circular or otherwise shaped foramen at or near its extremity, partly bounded by one or two plates, termed a deltidium.
A considerable length of the toga was allowed to hang from the left shoulder; the remainder was passed round the body so as to rise like a baldric (balteus) from the right hip to the left shoulder, being folded over in front (the fold was called sinus), then brought round the back of the neck so that the end fell over the right shoulder; the hanging portion on the left side was drawn up through the sinus, and bulged out in an umbo (Plate, fig.
The beak or umbo of each valve is prominent and rounded, and a number of sharp ridges and furrows radiate from the apex to the free edge of the shell, which is crenated.
They protected themselves in battle with a large wooden or wicker shield, the centre of which was ornamented with an iron boss (umbo).