_Scutum_ (woodcut 5): this valve is generally sub-triangular, and its three margins are the _basal_, the _tergal_, so called from being articulated with the tergum, and the _occludent_, so called from opening and shutting against the opposed valve. The angles are called from the adjoining margins, as _basi-tergal_, &c.; the upper angle being the apex. The scutum is ordinarily articulated to the tergum by an _articular ridge_ (_crista articularis_), running up to the apex of the valve, and by an _articular furrow_, which latter receives the scutal margin of the tergum. The articular ridge, instead of projecting straight up from the valve, when laid flat on its external surface, often bends over to the tergal side, and is then said to be _reflexed_. On the internal surface of the valve, there is almost always an _adductor pit_ or _cavity_ (_fossa adductoris_), for the attachment of the adductor scutorum muscle: this pit is often bounded on its tergal and basal sides, by a ridge, called the _adductor ridge_ (_crista adductoris_), which, in its upper part, is often confluent with the articular ridge. Beneath the adductor ridge, in the basi-tergal corner of the valve, there is often a _lateral-depressor pit_ (_fossa musculi lateralis depressoris_), for the attachment of the so-called muscle; and this pit is sometimes furnished with crests. _Tergum_, (woodcut 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, &c. 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.