BIVALVES. 
405 
some instances more or less inequivalve; and always remarkable for their straight 
hinge-line, furnished with very numerous teeth. The form is variable; but tho 
valves are generally radiately ribbed, and more or less covered with a periostracum, 
which may be smooth and thin, or thick, and very rugged. They may either meet 
all round when closed, or may gape ventrally for the passage of a byssus. There are 
two adductors far apart, and the pallial line is simple. The species—both recent and 
fossil—are very numerous ; and at the present time occur in all seas, some having a 
very wide distribution. For in¬ 
stance, the little Area lactea, 
which is found on the British 
coast, also occurs in the Philip¬ 
pine Islands, the Red Sea, South 
Africa, Ascension Island, and the 
Mediterranean; and another 
species ( A . corpulenta ) has been 
dredged off North Australia, 
south of Amboyna, in Mid-Pacific, 
and off the coast of Chili, in 
depths ranging from two hundred 
to two thousand four hundred 
and twenty-five fathoms. In the 
allied Pectunculus the shell is 
rounded, strong, equivalve, with 
the hinge-teeth in a curved line; 
the outer surface being sometimes 
covered with a velvety or pilose 
periostracum. Limopsis some¬ 
what resembles Pectunculus in 
form, but the shells are more com¬ 
pressed and clothed with a fibrous 
periostracum, and the animal 
spins a byssus. Several of the 
species have been dredged at 
enormous depths in the Atlantic. 
The genus Trigonia, represented 
by about half a dozen species 
occurring on the shores of 
Australia, is all that now remains of the large family Trigoniidce, of which several 
other genera, with a very large number of species, occur fossil in the Secondary 
and Tertiary rocks. The valves of Trigonia are beautifully pearly within, equal, 
radiately ribbed, with an external ligament, and a few strong striated divergent 
hinge-teeth. The umbones are inclined posteriorly—a very unusual feature in 
bivalves. The foot of the animal is large and powerful, used in crawling and 
leaping, and without a byssus. In some of the Jurassic rocks of Weymouth 
trigonias form a bed several feet in thickness. Mussels (family Mytilidce) are 
such well-known shells that a description is unnecessary. They are found all over 
common mussel [Mytilus edidis ), CLOSED and attached by the 
byssus (nat. size). 
