148 MOLLUSCA. LAMELLIBRANCHIATA. 



Thracia. Shell oblong, slightly compressed, atten- 

 uated and gaping posteriorly ; surface smooth, concen- 

 trically striated or somewhat granular. Right valve 

 larger than the left. Hinge with a cartilage-process in 

 each valve and a crescentic ossicle. External ligament 

 short and prominent. Adductor impressions small. Pal- 

 lial sinus deep. Trias to present day. 



Mactra. Shell oval, trigonal, nearly equilateral, 

 slightly gaping behind, smooth or with concentric striae. 

 Internal ligament in a large triangular pit. External 

 ligament in a groove. In front of the internal ligament 

 pit is a bifid cardinal tooth. Anterior- and posterior- 

 lateral teeth well-marked, compressed. Adductor im- 

 pressions semicircular. Pallial sinus round or angular. 

 Corallian to present day. 



Corbula. Shell oval, very inequivalve, closed, round- 

 ed in front, truncated behind. Surface generally with 

 concentric grooves. Umbones prominent. Right valve 

 larger and more convex than the left, and with a strong 

 cardinal tooth in front of the cartilage pit, and a posterior 

 cardinal tooth. Left valve with a projecting cartilage - 

 process. Adductor impressions well-marked. Pallial 

 sinus very small. Trias to present day. 



Pholas. Shell elongate, cylindrical, gaping at both 

 ends. Surface with spiny ridges, best marked in front. 

 Dorsal region with accessory valves. No teeth ; no liga- 

 ment. In the interior under the umbones is a process for 

 the insertion of the muscle of the foot. Pallial sinus very 

 deep. Lias to present day. 



Teredo. Shell more or less globular, gaping at the 

 ends, valves tri-lobed, with concentric striae. In the 



