28 CONCHOLOGY. 



P. costata. The ribbed Pholas. PI. 3, fig. 4. 



Elongated, wedge-shaped, covered with regular elevated 

 jagged or scalloped ribs, elegantly disposed; three dorsal 

 accessory pieces ; muscular impression almost medial. 



P. crispata. The curled Pholas. 



Somewhat oval, truncated behind, and as if divided into 

 two parts by an oblique furrow from the summit to the base ; 

 anterior part reticulated, the other parts plain ; muscular im- 

 pression marginal. 



P. clavata. The clubbed Pholas. 

 Short, wedge-form, little gaping, with many accessory 

 pieces. 



2. Gastrochffina. Three species. 



Always without accessory pieces, and, therefore, easily 

 distinguished from the Pholas. 



Shell equivalve, somewhat wedge-shaped, with a very 

 large, oval, oblique, anterior opening between the valves ; 

 the posterior extremity nearly close ; hinge linear, marginal, 

 without teeth ; two distant muscular impressions ; some- 

 times with a kind of tube or calcareous general envelope. 

 Gastrochsena cuneiformis. Gastrochaena mytiloides. 

 G. modiolina. 



G. modiolina. The Modioliform Gastrochaena. 

 Oval, thin, brittle, gaping at the side ; light reddish brown, 

 with a bluish white interior. 



G. cimeiforjnis. The wedgelike Gastrochaena. PI. 3, 

 fig. 1. 



Species with a smooth shell, and without distinct tube. 

 (Represented as imbedded in wood.) 



