100 CLASS ACEPHALA. 
valves are about equal, and of a fibrous texture. ‘They ap- 
pear to have had a byssus. 
PULVINITES, Defr., 
A regularly triangular shell, in which the few depressions 
diverge from the summit on the inside. The impression is 
found in chalk. 
In the second subdivision of the ostracea, as well as in 
almost all the bivalves which follow, besides the single trans- 
verse- muscular mass of the preceding genera, there is a 
bundle which is placed before the mouth, and extends from 
one valve to the other. It is apparently in this subdivision 
that we must place the 
ETHERIA, Lam., 
Large inequivalve shells, as irregular as those of the ostrex, 
and more so; no teeth to the hinge; the ligament partly 
external and partly internal. They differ from the ostrez 
chiefly in having two muscular impressions. The animal is — 
not seen to produce a byssus. 
They have lately been discovered in the upper Nile. 
AVICULA, Brug., 
An equivalve shell with a rectilinear hinge, frequently ex- 
tended into wings, at its extremities, furnished with a narrow 
and elongated ligament, and sometimes with small denticula- 
tions near the mouth of the animal. In the anterior side, 
a little beneath the angle of the side of the mouth, is a small 
notch for the byssus. The anterior transverse muscle is still 
excessively small. 
The species with less salient ears form the PINYADINAE, 
Lam., or MARGARITA, Leach. 
The most celebrated (myiilus margaritiferus, Lin.) has 
nearly a semicircular shell, greenish without, and ornamented 
