410 SCROBICULARIIN^;. 



Genus ABRA, Leach. 



Shell thin, transversely elongated, slightly gaping at the 

 sides ; surface smooth, covered with a thin, deciduous epi- 

 dermis. Hinge with an oblique cartilage-pit for the internal 

 cartilage; primary teeth small, or wanting; lateral teeth 

 distinct ; ligament short, partly external. Muscular im- 

 pressions rounded ; pallial line deeply and widely sinuated. 



Syn. Erycina, Lam., not Recluz. Syndosmya, Recluz. 



Ex. A. intermedia, Thompson, pi. 105, fig. 4. Shell, 

 A. tenuis, Montagu, fig. 4, a, 4, b. 



The species of Abra are few, and mostly boreal in their 

 geographical distribution. They range from the laminarian 

 zone to one hundred arid eighty fathoms, living buried in 

 sand and mud. The species are found in Norway, Britain, 

 the Mediterranean, and the Black Sea. 



Species of Abra. 



alba, Wood. segmentina, Recluz. 



intermedia, Thomp. tenuis, Mont. 



prismatica, Mont. 



Genus SEMELE, Schumacher. 



Shell transversely oval or orbicular, slightly gaping at 

 the sides. Hinge with one or two primary teeth in each 

 valve, with a long, narrow pit between them for the internal 

 cartilage; ligament external, thin. Muscular impressions 

 round ; pallial line with a wide and deep sinus. 



Syn. Amphidesma, Lamarck. 



Ex. S. reticulata, Sowerby, pi. 105, fig. 5, 5, a. 



