520 MODIOLARCIDiK. 



anal opening moderate, plain ; pedal aperture small, basal, 

 sub-anterior. Gills four, thick, dependent, sub-trigonal, 

 truncated in front, narrow, produced and united behind. 

 Foot with a small flat sole crenulated at the edge, deeply- 

 grooved behind, and byssiferous. 



Shell equivalve, covered with a hard, polished epidermis. 

 Hinge-teeth none, or rudimentary; cartilage linear, external. 



Living attached to floating sea-weeds. 



Genus MODIOLARCA, Gray. 



Shell ovate, trapezoidal, thin, fragile, ventricose ; surface 

 of valves covered with a smooth, shining epidermis; beaks 

 anterior, prominent, contiguous ; anterior end compressed, 

 sloping abruptly to the base; hind margin rounded, ven- 

 tral margin somewhat sinuous and gaping anteriorly. 

 Hinge with two small, oblique teeth in the right valve, 

 which receive two corresponding ones of the left. Mus- 

 cular impressions distinct; pallia! sigmoid. 



Syn. Phascolicama, Valenc. Gaimardia, Gould. 



Ex. M. trapezina, Lamarck, pi. 122, fig. 1, 1, a. 



The genus Modiolarca resembles the Palaeozoic Modio- 

 lopsis in the large size of the anterior adductor impression. 

 M. trapezina is found attached by its byssus to floating 

 sea-weed in many parts of the Southern Ocean, as at 

 the Falkland Islands, and off Kerguelin; and M. subtorta 

 on the north coast of New Holland. 



Species of Modiolarca. 

 subtorta, Dkr trapezina, Lam. 



