Mactra.} PELECYPODA. %5 



Hab. North and South Islands, from the Bay of Islands to Pre- 

 servation Island ; washed up on sandy beaches after heavy gales. 

 Maori. Whangaikararo (teste Captain Bollons). 

 Fossil. -Miocene and Pliocene. 



Subgen. 3. MACTROTOMA, Dall, 1894. 



Mactrotoma, Dall, " Nautilus," viii, 1894, 26 ; Trans. Wagn. Free lust., iii, 

 pt. 4, 876. Type: Mactra fragilis, Gmel. (= brasiliana, Lam.)- Stundella, 

 H. and A. Adams, 1856 ; not of Gray, 1853. 



Shell subequilateral, elongate ; with a thin, silky epidermis, pos- 

 terior dorsal areas bordered by an impressed fascicle over which the 

 epidermis is darker-coloured and differently wrinkled ; beaks ad- 

 jacent ; pallial sinus large ; valves convex, gaping markedly ; liga- 

 ment lanceolate ; choiidrophore large, shallow, apically roofed ; 

 anterior laminae issuing from the dorsal sinus ; cardinals prominent, 

 thin, their posterior arms projecting over the choiidrophore ; each 

 anterior arm attended by a high accessory lamella in nearly the same 

 plane, closely appressed in the right valve to the ventral lamina, 

 and in the left valve to the anterior lateral, so that to a cursory 

 inspection the lamina appears tridentate and the tooth bidentate. 



KEY TO SPECIES. 



A. Shell elongated oval, subtrigonal, compressed, solid . . . . elongata. 



B. Shell ovate, ventricose, thin, with fine concentric striation . . ovata. 



C. Shell suborbicular, compressed, beaks almost median, rudely con- 



centrically striated . . . . . . . . . . rudis. 



3. Mactra elongata, Quoy and Gaimard, 1835. Plate 60, fig. 2. 



Mactra elongata, Q. & G., Voy. Astrol., iii, 1835, 518, pi. 83, f. 1, 2. Sptsula 

 elongata, Q. & G. : Gray, A.M.N.H., i, 1837, 271 ; Dieff. N.Z., 251. 

 Mactra (Standella) elongata, Q. & G., Crit. List, 44. Standella elongata, 

 (). & G. : Button, J. de Conch., xxvi, 46 ; M.N.Z.M., 139 ; Index, 90. 

 Hemimactra elongata, Q. & G. : Hutton, P.L.S. N.S.W., is, 518. Mulinia 

 notata, Hutton, C.M.M., 64. Standella notata, Hutt., M.N.Z.M., 139. 

 Hemimactra notata, Hutt., P.L.S. N.S.W., ix, 518. 



Shell elongated oval, a little trigonal, lightly convex, inequilateral, 

 concentrically striated and ribbed, light brown with irregular dark- 

 brown spots and dashes. Beaks approximate, not much raised, turned 

 forwards. Anterior end shorter, narrowly rounded, the dorsal margin 

 a little excavated in front of the umbones, then slightly convex and 

 slowly descending ; posterior end narrowly rounded and subtruncated 

 towards the base, the dorsal margin lightly convex, descending ; 

 basal margin broadly rounded. Sculpture consisting of very fine con- 

 centric striae, rugose ribs towards the base and on the posterior area. 

 Epidermis thin, silky. Colour yellowish - brown with dark - brown 

 irregular spots and dashes. Interior yellowish-white, porcellanous, 

 polished, with faint radiate striation. Margins smooth, solid. Hinge : 

 Right valve with 2 short diverging cardinals, the posterior very short 

 and extending over the resilifer, a short accessory lamella in front of 



