964 PELECYPODA. [Eulamellibranchia. 



dental armature concentrated, teeth and laminae short, the anterior 

 arm of the right cardinal lying in the plane of the ventral lamina ; 

 otherwise as in Mactra. 



Sect. 1. CYCLOMACTRA, Dall, 1895. 



Cydomactra, Dall, P. Mai. S., i, 1895, 211 ; Trans. Wagn. Free Inst,, iii, 

 pt. 4, 875. Type : Mactra tristis, Gray. 



Shell subcircular, compressed, pedal gape obsolete, ligament sub- 

 merged except the tip, but wholly separated from the resihum ; 

 remainder of characters like Mactroderma. 



Australian seas and New Zealand. 



2. Mactra discors, Gray, 1837. Plate 60, figs. 1, a. 



Mactra discors, Gray, A.M.N.H., i, 1837, 371 ; Conch. Icon., viii, f. 17 ; 

 Dieff. N.Z., 251 ; C.M.M., 63 ; Crit. List, 43 ; Ereb. & Ter., 5, pi. 2, 

 f. 4; Hutton, J. de Conch., xxvi, 45; M.N.Z.M., 138; P.L.S. N.S.W., ix, 

 517. M. Murchisoni, Deshayes, P.Z.S., 1854 (1855), 64; Conch. Icon., 

 viii, f. 76 ; C.M.M., 63 ; Crit. List, 43 ; M.N.Z.M., 138. 



Shell rather large, rotundately ovate, somewhat triangular, rather 

 thick, subequilateral, regularly convex, posterior slope depressed, 

 linearly keeled, yellowish-white. Beaks small, approximate, directed 

 forwards, smooth, white. Anterior end somewhat shorter, compressed, 

 convex, the dorsal margin straight, descending ; posterior end flattened 

 above, broadly convex, surrounded with a slightly keeled obtuse 

 angle, slightly angularly produced below ; basal margin regularly 

 broadly convex. Sculpture consisting of fine growth-lines produced 

 into sharp oblique riblets on the lunular area, and faint radiate striee. 

 Epidermis yellowish or light brown, wrinkled, thin, usually only 

 present near the margins. Colour greyish-white, yellowish or fulvous 

 where covered by the epidermis. Interior white, porcellanous, shining, 

 faintly radially striated. Margins smooth and sharp, sometimes with 

 light radiate strise. Hinge : Eight valve with a median A -shaped 

 cardinal, both arms of equal length, posteriorly with 2 accessory 

 lamellae, the anterior coalescent with the cardinal, the second more 

 free ; 2 lamellae on each side, which are rather short, the lower ones 

 higher than the others ; left valve with 2 cardinal lamellae which are 

 thin, subparallel, and short, with a posterior accessory lamella which 

 in adult shells is often obsolete ; lateral lamellae short and high ; 

 resilifer triangular, with an apical roof. Ligament lanceolate, inset, 

 short, behind the umbones, not connected with the resilifer. Ad- 

 ductor-scars subequal, not very deep, pyriform. Pallial sinus short, 

 broadly rounded above, narrowly in front. 



Length, 92 mm. ; height, 80 mm. ; diameter, 47 mm. (large 

 specimen). 



Type in the British Museum. 



