58 The Shell-Collector’'s Handbook. 
DREISSENA POLYMORPHA, PALLAS. 
SHELL mussel-shaped, triangular, keeled in centre of both 
valves, flattened below, rather solid, olive or yellowish- 
brown, marked transversely on the upper part with zigzag 
streaks of purple or dark brown; umbones incurved, 
small, and placed at the anterior end; ligament long, 
narrow, and fitting into a sulcus in the hinge of each 
valve; hinge strong, toothless, with a hollow triangular 
plate under the umbones in both valves ; upper margin 
angular ; anterior margin nearly straight; posterior side 
curved; lower margin incurved. Animal blackish, fur- 
nished with a byssus. 
Habitat.—Canals, lakes and rivers. 
