then regularly rounded, posterior broader than anterior, horizontal above, then very obliquely truncated, ventral margin with broad shallow sinus; rounded ridge bounding posterior area runs from umbo to posterio-ventral corner. Sculpture of about 12 primary ribs which by anastomosing produce about 24 at adult margin; these are crossed by very strong regular concentric growth-ridges raised on radials into strong scales. Teeth 6 to 8 anterior and 10 to 12 posterior, not separated by smooth space. Area very narrow, not well defined anteriorly, one strong groove bounding the area running from beak to end of hinge-line, but seldom any others and then they are quite short. Valve-margin crenulated.

Height 4 mm.; length 7 mm.; thickness (1 valve) 2 mm.

Locality: Titirangi.

Apparently closely related to Arca sociella Brookes from North Auckland and to Arca botanica Hedley from southern and eastern Australia, but with fewer ribs than either of these.