534 ARCING. 



maculata, Sow. Pacifica, Sow. 



mutabilis, Soiv. tetragona, Poll. 



navicularis, Brag. truncata, Sow. 



Nose, Linn. volucris, Beeve. 



ocellata, Beeve. zebra, Swains. 



Sub-gen. lithakca, Gray. 



Shell wedge-shaped, obliquely truncated behind, elongated and 

 rounded in front. 



lithodomus, Sow. 



Genus BARBATIA, Gray. 



Shell oblong, longitudinal, or sub-quadrate ; surface of 

 valves covered with an epidermis which is usually loose 

 and rough. Hinge with the teeth .curved, divided into 

 broad, more or less oblique, or sub-conic plates dilated at 

 the outer end; ligament external; ligamental area narrow, 

 cartilage grooves angular, concentric. Pallial impression 

 entire. 



Fa:. A barbata, Linmeus, pi. 124, fig. 4. Shell, A. 

 barbata, fig. 4, a, 4, b. 



The species of this genus are numerous, and very widely 

 distributed ; a few are inhabitants of the Arctic regions 

 and the coasts of Britain. They, like the Arcs, attach 

 themselves by a byssus to rocks and stones. 



Species of Barbatia. 



barbata, Linn. decussata, Sow. 



cometa, Reeve. fasciata, Beeve. 



cruciata, Phil. fusca, Brug. 



