The keel is pushed back to the distal third of the sternum, whilst the original anterior margin of the keel is correspondingly elongated,and the furcula fused with the rostral portion.
The central and umbonal muscles effect the direct opening and closing of the shell, the laterals enable the valves to move forward and backward on each other, and the transmedians allow the similar extremities (the rostral) of the valves to turn from each other to the right or the left on an axis subcentrically situated, that is, the medio-transverse region of the dorsal valve.
Almost always the shell has a rostral sinus.
In these the furcal branches are linear or rudimentary, the shell is without rostral sinus, and, besides distinguishing characters of the second 2ntennae, they have always a branchial plate well developed on the first maxillae, which is inconstant in the other tribe.
2), or membranaceous and polished, hairy or smooth, oval or round or bean-shaped, or of some less simple pattern; the valves may fit neatly, or one overlap the other, their hinge may have teeth or be edentulous, and their front part may be excavated for the protrusion of the antennae or have no such " rostral sinus."
The dorsal crest is continued round both extremities, becoming expanded to form the rostral fin in front and the caudal fin behind.
In this connexion it may also be mentioned that in all cases the right half of the oral hood is directly continuous with the rostral fin (fig.
(Spence Bate.) r, Rostral spine.