968 PELECYPODA. [Eulamellibranchia. 



Subgen. 1. SPISULA, s. str. 



1. Spisula ordinaria, E. A. Smith, 1898. Plate 63, fig. 16. 



Mactra ordinaria, Smith, P. Mai. S., iii, 1898, 23, f. vii in text ; Index, 90 ; 

 Suter, T.N.Z.I., xxxviii, 319. M. triangulare, Hector, Cat. Colon. 

 Museum, 1870, 173 ; not of Lamarck. M . elegans, Hutton, C. Tert. M., 

 19 ; not of Sowerby. 



Shell small, thin, triangularly ovate, inequilateral, horn-colour, 

 the valves somewhat ventricose, the dorsal areas angled at the sides- 

 Beaks small, but little raised, incurved, approximate. Anterior end 

 shorter, convex, the dorsal margin descending, almost straight ; 

 posterior end angularly rounded, the dorsal margin long, lightly convex, 

 descending ; ventral margin broadly rounded. Sculpture consisting 

 of very fine and close concentric strife, produced into sharp riblets 

 on the dorsal areas. Epidermis thin, greyish-white, lightly shining. 

 Colour brownish-grey. Interior whitish, with faint radiate strife. 

 Margins sharp, smooth. Hinge : Eight valve with two cardinals, 

 adjacent above, the posterior tooth shorter, parallel to the margin 

 of the resilifer, the anterior and upper cardinal longer, close to the 

 dorsal valve-margin ; the 2 anterior and 2 posterior lamellse rather 

 long, sharp, convexly raised ; left valve with a A-shaped cardinal 

 and a short accessory lamella behind, which, together with the posterior 

 arm of the cardinal, extend over part of the resilifer ; the lateral 

 laminae long, convexly raised ; resilifer small, obliquely triangular. 

 Ligament saggitate, small, extending to the apically open resilifer. 

 Adductor-scars unequal, the anterior smaller, pyriform, the posterior 

 roundish. Pallial sinus broad, not deep, broadly rounded in front. 



Length, 13 mm. ; height, 9-5 mm. ; diameter, 5-5 mm. (type). 



Type in the British Museum. 



Hob. Lyttelton Harbour, in 4 fathoms, type (H. S.) ; Auckland 

 Harbour; off Wanganui, in 20 fathoms ; Cook Strait ; Blind" Bay. 



Fossil in the Pliocene. 



Subgen. 2. HEMIMACTKA, Swainson, 1840. 



Hemimactra, Swainson, Treat. Malac., 1840, 369 ; Dall, Trans. Wagn. Free 

 Inst., iii, pt. 4, 878. Type : Mactra solidissima, Dillvvvn. 



Shell large, ovate-trigonal, with grooved laterals and rather con- 

 centrated hinge ; the dorsal areas are not grooved, and the anterior 

 arm of the right cardinal is confluent with its ventral lamina ; car- 

 dinals markedly compressed. (Dall.) 



Sect, 1. OXYPERAS, Morch, 1853. 



O.ri/iirms, Morch, Cat. Conch. Yoldi, 1853, pt. 2, 4 ; Dall, Trans. Wagn. 

 Free Inst., iii, pt. 4, 879. Type : Mactra striatella, Lamarck. 



This section is characterized by its more crudely triangular shape 

 and rather strong concentric sulcation of the Surface of the valves. 

 It is chiefly Lido-Pacific in its distribution. (Dall.) 



