384 



American Se ash ells 



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- 



