Mactra.] PELECYPODA. 963 



sagittate, linear, completely shut off from the chondrophore by shelly 

 matter ; dental armature not concentrated ; laterals long, thin, 

 and flexuous, distally confluent with the hinge-plate margin, as is the 

 anterior ventral 'lamina ; anterior sinus roofed by a buttress, upon 

 which stands the anterior arm of each cardinal, and from under which 

 the laterals emerge ; chondrophore roofed at the apex ; right car- 

 dinal not coalescent above, the anterior arm adjacent to the dorsal 

 shell-margin ; hinge-plate very oblique, especially in front, forming 

 a deep recess extending to the beaks. 



1. Mactra scalpellum, Reeve, 1854. Plate 63, fig. 14. 



Mactra scalpellum, Reeve, Conch. Icon., viii, 1854, pi. 19, f. 106 ; Ueshayes, 

 P.Z.S., 1854 (1855), 65 ; C.M.M., 63 ; Grit. List, 43 ; Button, J. de 

 Conch., xxvi, 45 ; M.N.Z.M., 138 ; P.L.S. N.S.W., ix, 517 ; Murdoch, 

 T.N.Z.I., xxxii, 221, pi. 20, f. 10 ; Hedley, T.N.Z.I., xxxviii, 75. Darina 

 pusilla, Hutton, C.M.M., 64. 



Shell small, triangularly oblong, thin, compressed, equilateral, 

 smooth, shining, yellowish-white. Beaks produced, acute, approxi- 

 mate. Anterior end narrowly rounded, the dorsal margin straight, 

 slowly descending ; posterior end subangular or very narrowly convex, 

 the dorsal margin very little arched, descending at the same angle 

 as the anterior ; basal margin broadly convex ; an indistinct ridge 

 descending posteriorly from the umbo. Sculpture consisting of very 

 fine and faint concentric strise, produced into sharp riblets on the 

 dorsal areas. Epidermis thin, shining. Colour yellowish - white. 

 Interior bluish-white, polished, smooth. Margins simple, sharp. 

 Hinge : Right valve with the anterior cardinal long, oblique, descend- 

 ing to the lower lateral tooth, the posterior cardinal short, directed 

 towards the apex of the resilifer, 2 anterior and 2 posterior long 

 laterals ; left valve with a hooked cardinal in front of the resilifer, 

 the posterior arm much shorter, 1 anterior and 1 posterior long 

 lateral ; resilifer triangular, large, roofed above. Ligament small, 

 sagittate, close behind the beaks, not connected with the resilium. 

 Adductor-scars subequal, large, not deep. Pallial sinus moderately 

 large, high, rounded in front. 



Length, 24 mm. ; height, 15 mm. ; diameter, 6-5 mm. 



Type in the British Museum. 



Hob. New Zealand (Strange) : near Cuvier Island, in 38 fathoms 

 (Captain Bollons) ; ofE Great Barrier Island, in 110 fathoms ; near 

 Little Barrier Island, in 20 fathoms ; near Channel Island, Hauraki 

 Gulf, in 25 fathoms. 



Fossil in the Pliocene. 



Subgen. 2. MACTRODERMA, Ball, 1894. 



Mactroderma, Dall, " Nautilus," viii, 1894, 39 ; Trans. Wagn. Free Inst., iii, 

 pt. 4, 875. Type : Mactra velata, Philippi. 



Shell inequilateral, rude, with a coarse epidermis, pronounced 

 pedal gape, a lanceolate sunken ligament, an inconspicuous spur ; 



31* 



