270 TELLINIDiE. 



Soleciirtida ; through ^yndosmya and Scrohicularia with 

 the Mactrada. 



PSAMMOBIA, Lamarck. 



Shell transversely oblong, equlvalve, subinequilateral, 

 slightly gaping at the extremities ; surface smooth, or 

 transversely, and more or less radiatingly striated, invested 

 with a thin epidermis. ^luscular impression round ; pal- 

 lial sinus strongly marked. Hinge composed of cardinal 

 teeth, two or a single bifid tooth in 0)ie valve, and one in 

 the other ; supplementary laminse small, and often obso- 

 lete. Ligament prominent, external. 



The animal is oblong and compressed ; its mantle is 

 open throughout its length, and bordered by a fringe of 

 fine simple filaments ; the siphons are very long, slendei-, 

 and delicate, marked with longitudinal ciliated lines, which 

 terminate in more or less conspicuous cirrhi, few (six or 

 eight) in number, surrounding their orifices. The foot is 

 rather large, and linguiform. The labial tentacles are tri- 

 angular, and internally pectinated. One of the branchial 

 leaflets on each side is shorter than the other. 



The PsammohirE are inhabitants of most seas, though 

 sparingly distributed, and may be traced, though doubt- 

 fully, far back in time. They live buried in sand or 

 gravelly mud, some in the littoral, some in the laminarian 

 and coralline zones, and several species have a considerable 

 range in depth. Our native examples are all shells 

 remarkable for elegance and beauty. The animals are not 

 of dull and torpid habits, but active in their motions, 

 though moving about below the surface of the sea-bed. 



