_Scuta_, slightly convex, oblong, breadth about two thirds of the length, almost quadrilateral, with the upper portion produced into a flat projection; this projection is almost spear-shaped, being constricted a little on each side below the apex. There is a deep pit for the adductor muscle. The umbo is near the apex, the part above not being above one fifth of the whole length of the valve. As in _S. vulgare_, the growth is at first downwards, and subsequently a little upwards and downwards, thus producing the upper, small, spear-like projection, which lies at a lower level than the umbo. There is a fold on the occludent margin. _Terga_, large, flat, triangular; carinal margin slightly hollowed out; occludent margin slightly arched, with a small portion protuberant to a variable amount. The apex is slightly curved towards the carina. _Carina_, long, internally deeply concave, angularly bent, the lower portion slightly longer and wider than the upper part; the two halves meet each other at about an angle of 135°; the upper half is parallel to the longer axis of the terga, between which it extends for three fourths of their length. The external surface is rounded, except near the umbo, where the edge is carinated; growth almost equally upwards and downwards; the parietes and tectum are not separated by ridges. The _Sub-carina_ lies close under the carina, and is placed almost transversely to the longer axis of the capitulum; external surface arched and smooth, the whole having the shape of half of a cone, with the apex a little curved outwards; seen internally, it may be said to be formed of two triangular wings placed at right angles to each other; basal margin straight; in size equalling the carinal latera. _Rostrum_, lying almost transversely to the longer axis of the capitulum, under the basal margins of the scuta; in shape (fig. 6 _a_) closely resembling the sub-carina, but about one third larger than it; larger also than either the rostral or carinal latera; seen externally, appears like a half cone; seen internally, is formed of two triangular wings (with curved edges), placed at right-angles to each other. _Upper Latera_, internally flat, oblong, twice as long as broad; upper end square, truncated; upper half rather wider than the lower half; fully twice as large as either of the lower latera. The basal points extend below the basal margins of the scuta. The umbo is placed a little above the centre. _Rostral Latera_, minute, scarcely exceeding one third of the size of the carinal latera, and very much less than the rostrum; they are placed transversely under the basal point of the upper latus, or rather between it and the baso-lateral angle of the scutum; basal margin, as seen internally, straight; upper margin arched; rostral angle produced; internally flat; the whole valve is very thick and solid, so that the umbo which lies at the rostral end, projects rectangularly outwards.