BY K. M. JOHNSTON, F.L.S. 



113 



(xcii.) Family NiTcr/MD*. 

 Shell oval, or trigonal, small, nacreous within; hinge composed 

 of a great number of transverse teeth, interrupted by a 

 central pit for the reception of the ligament, which is 

 internal or external. 



Genus Nttcula. (673-674) 

 Shell trigonal, with the umbones turned towards the short 

 posterior side, smooth or sculptured ; epidermis olive, 

 interior pearly ; margins cre7iulated ; hinge with promi- 

 nent internal cartilage pit, and a series of sharp teeth 

 on each side ; pallial line simple. Dist. — Northern 

 and Arctic Seas; 10 — 180 fathoms; Siberia, Melville 

 Island, New England, Britain, Mediterranean, Cape, 

 Japan, Australia, 



Genus Lbda. (675-676.) 

 Shell resembling Nucula ; oblong, rounded in front, produced 

 and pointed behind ; margins even ; pallial line with a 

 small sinus ; umbonal area with a linear impression 

 joining the anterior adductor. Dist. — Northern and 

 Arctic Seas; 10 — 180 fathoms; Siberia, Melville 

 Island, New England, Britain, Mediterranean, Cape, 

 Japan, Australia. 



Shell 





(xciii.) Family Abctd^. 

 regular, equivalve, with strong epidermis ; ligament 

 exterior, occupying an area between the beaks ; hinge 

 with a long row of similar comb-like teeth ; pallial line 

 distinct ; muscular impressions sub-equal ; structure 

 corrugated, with vertical tubuli in rays between the 

 ribs of striae. 



Genus Arca. (677-681.) 

 knell equivalve, or nearly so, thick, sub-quadrate, ventricose, 

 strongly ribbed or cancellated ; margins smooth or 

 dentated, close or sinuated ventrally ; binge straight, 

 teeth very numerous transverse ; umbones anterior, 

 separated by a flat, lozenge-shaped ligamental area, 

 with numerous cartilage grooves ; pallial line simple ; 

 posterior adductor impression double ; pedal scars two, 

 the posterior elongated. Dist. — World-wide, most 

 abundant in warm seas; low-water, 230 fathoms. 



Genus Pectttnculus. (682-685.) 



ell orbicular, nearly equilateral, smooth or radiately striated ; 



umbones central, divided by a striated ligamental 



area; hinge with a semi-circular row of transverse 



teeth ; adductor sub-equal ; pallial line simple ; 



H 



