Gastrochaenidae and Pholadidae 93 



edentulous, and there is an external ligament, two unequal muscular 

 impressions and a deep pallial sinus. 



Viewed separately the valves are seen to be twisted from plane 

 of the long axis. It is this torsion which produces the extreme anterior 

 gaping. 



Genus GASTROCHAEIVA Spengler. 1783 

 Gastrochaena ovatai*^^ Sowerby PI. 19, figs. 123a, b 



Alt., 5.75; length, 12 mm. Shell narrows rather sharply toward 

 posterior extremity; concentric sculpture of closely placed, slightly 

 lamellate threads, not equidistant. Interior of valves with definite 

 central, longitudinal ridge. 



Burrows often found m shells m Spondylus and Plicatula. 



Gastrochaena stimpsonil (Tryon) PI. 44, figs. 311a, b 



Alt., 6.25; length, 13 mm. Anterior border a little reflected 

 outward causing a notched appearance; surface sculpture only of 

 incremental lines; no mterior rib. 



Usually found in coral. 



Family PHOLADIDAE 



The Pholadidae is a family of penetrating moUusks. Its distri- 

 bution is through all seas, and some of its members are able to pen- 

 etrate any substance softer than their own shells. When once estab- 

 lished in its burrow, the essential circulation of sea water is main- 

 tained through the mollusk's two large siphons. 



The shells are white, thin and brittle, with thin epidermis; much 

 elongated, generally narrowed toward the posterior end, equi- 

 valve, gaping at one or both ends. The umbos are inconspicuous, 

 rounded and placed anteriorly. The anterior dorsal margin is more or 

 less reflected over the umbonal region. There is no well-defined 

 ligament, but a posterior accessory plate is present. The sculpture 

 consists principally of prominent ribs and stnations. The hinge is 

 edentulous. Each valve has an internal apophysis in the umbonal 

 region. Thin, membranous ligaments unite the valves anteriorly, and 

 posteriorly below the accessory plate. 



^''■* Gr., gaster, stomach; chfino, gape or yawn; Lat., ovatus, from ovum, 

 egg. 



