BIVALVES. 21 
adductor muscle is supported on a triangular 
shelf below the beak. 
The mantle of Driessena is united all round, with 
the exception of three apertures (fig. 2)—one the 
anal orifice, which is prolonged into a very small 
tube; a second, the branchial orifice, furnished 
with a prominent siphon or tube, which is fringed 
on the inside. 
The Zebra Mussel is an attached bivalve, like 
its brethren of the sea; the foot (f) is very im- 
perfectly developed, giving 
place to a gland, which se- 
cretes the material of the # 
threads with which it attaches (“ZZ 
itself to stones, timber, and 
shells ; these threads are 
termed the byssus (b); and pig, 9. —Driessena 
the third opening in the united PART 
mantle lobes is for the passage of these mooring 
cables. The epidermis of the shell is yellowish- 
brown, with undulating streaks, or zebra-like 
markings of dark brown. 
This mussel was first discovered by Pallas, in 
the different rivers of Russia and also in the 
Caspian Sea; and from the great variety of 
forms presented by this species, he designated 
it under the name Mytilus polymorphus. Van 
Beneden, in 1835, showed it to differ from Mytilus, 
and constituted for it the genus Driessena, de- 
