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384 Subfamily THYASIRINAE Genus Thyasira Lamarck 1818 Shell subglobular and of an earthy texture; umbones directed forward; posterior region of valve deeply furrowed; lunule absent; ligament in a groove and partly external; hinge without teeth and indented in front of the umbo; pallial line without a sinus. Thyasira trisinuata Orbigny Atlantic Cleft Clam Nova Scotia to south half of Florida and the West Indies. /4 to /4 inch in length, oblong, fragile and translucent-white. Hinge weak and with only a very long, weak posterior lateral. Posterior slope of shell with 2 strong, radial waves or rounded grooves. Moderately common in dredgings from 15 to 90 fathoms on sandy bottom, Thyasira gouldi Philippi (Labrador to off North Carohna) is similar, but only % inch in size, almost round but slightly higher, and with a weak yel- lowish periostracum. The hinge lacks teeth. Common offshore to 60 fath- oms. Called Gould's Cleft Clam.

Figure 77. Pacific Cleft Clams, a, Thyasira bis e eta Conrad, i inch (Pacific Coast); b and c, Thyasira excavata Dall, % inch (Gulf of California). Thyasira bisecta Conrad Pacific Cleft Clam Figure 77a Alaska to Oregon. I to 1/4 inches in length, almost square in side view and moderately obese. Characterized by the almost vertical, straight, anterior end which is 90 degrees to the dorsal margin. Ligament long and narrow and flush with the dorsal margin of the shell. There is a deep, prominent radial furrow on the exterior running posteriorly from the beaks. Shell chalky-white, com-