In the Trimerellidae, for example, some of the muscles are attached to a massive or vaulted platform situated in the medio-longitudinal region of the posterior half or umbonal portion of both valves.
Teriors; k, middles; 1, g, Umbonal muscular impresoutsiders), enabling the sions (open valves).
King makes out five pairs and an odd one, and individualizes their respective functions as follows: - Three pairs are lateral, having their members limited to the sides of the shell; one pair are transmedians, each member passing across the middle of the reverse side of the shell, while the odd muscle occupies the umbonal cavity.
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.
I [i]) is called the umbonal area; the great anterior muscular surface h is that of the anterior adductor muscle, the posterior similar surface i is that of the posterior adductor muscle; the long line of attachment u is the simple " pallial muscle," - a thickened ridge which is seen to run parallel to the margin of the mantle-skirt in this Lamellibranch.
'crin e' L ore to the shell but to sever also a strong elastic ligament, or spring resem bling india-rubber, joining the two shells about the umbonal area.
On the other hand, the ligament h acts upon the short arm formed by the umbonal ridge of the shells; whenever the adductors relax, the elastic substance of the ligament contracts, and the shells gape.
In Pholas dactylus we find a pair of umbonal plates, a dors-umbonal plate and a dorsal plate.