98 CLASS ACEPHALA. 
PLICATULA, Lam. 
The plicatula, separated by Lamarck from the spondyli, 
have nearly the same kind of hinge, but no heel, and flat, 
almost equal, irregular, plicated, and scaly valves, as in many 
of the ostrez. 
MALLEUS, Lam. 
A simple depression for the ligament, as in the ostrex, 
where the mallei were left by Linneus on account of their 
having the same irregular and inequivalve shell; but they are 
distinguished by a notch on the side of this ligament for the 
passage of a byssus. 
The most known species, ostrea malleus, Lin., which ranks 
among the number of high priced and rare shells, has the two 
ends of the hinge extended, and forming something like the 
head of a hammer, of which the valves, elongated in a trans- 
verse direction, represent the handle. It inhabits the Archi- 
pelago of India. 
There are some others, possibly young ones of the same 
species, in which the hinge is not prolonged: we must be 
careful not to confound them with the vulselle. 
VULSELLA, Lam., 
A little plate projecting inwards at each side of the hinge, 
from one of which to the other extends the ligament; other- 
wise similar to that of the ostree. By the side of this plate 
is a notch for the byssus, as in the mallei. The shell is 
elongated in a direction perpendicular to the hinge. | 
The most known species inhabit the Indian Ocean. 
PERNA, Brug., 
Several parallel cavities across the hinge, opposed to each 
ts —_— _— 
al i 
