[11]

Ter gam (figs 6 and 7): this valve, also, has three margins, the scutal, basal, and carinal; its upper end, or apex, is sometimes beaked; on the basal margin a spur (calcar) depends; the outer surface of the valve is depressed or longitudinally furrowed (sulcus longitudinalis) in the line of the spur. The part called the spur is often so broad, that the name becomes not very appropriate. The angles are denominated, from the adjoining margins, as basi-carinal, or basi-scutal angle, etc. On the under­ side, in the upper part, there is an articular ridge, and on its scutal side, an articular furrow, receiving the articular ridge of the scutum. In the basi-carinal corner of the valve there are often crests for the attachment of the tergal depressor muscle.