Placunanomia.] PELEC'YPODA. 845 



Genus 2. PLACUNANOMIA, Broderip, 1832. 



Placunanomia, Broderip, P.Z.S., 1832, 28. Type : P. Cumingi, Broderip. 

 Placunomia, Swainson, 1840. 



Shell having the appearance of Anomia ; left valve with 2 tangential 

 muscular impressions towards the centre the upper one, radially 

 striated, is a byssal scar, and the lower one appertains to the adductor 

 of the valves ; right valve perforated, the processes soldered together ; 

 elastic ligament on 2 diverging lamella? on the right side ; the cal- 

 careous plug sometimes soldered to the margins of the orifice of the 

 right valve. 



Distribution. European seas, Antilles, North Pacific, Austral- 

 asia, &c. 



Fossil in the Tertiary of Australia and New Zealand. 



Vernacular Name. Saddle-oyster. 



1. Placunanomia zelandica, Gray, 1843. Plate 56, figs. 1, la. 



Anomia zelandica, Gray, Dieff. N.Z., 260. Placunanomia zelandica, Gray, 

 P.Z.S., 1849 (1850),' 123; Button, C.M.M., 84; M.N.Z.M., 174; P.L.S. 

 N.S.W., ix, 532. Anomia (Placunanomia) zelandica, Gray, Ereb. & Ter., 

 7, p). 3, f. 10 ; Conch. Icon., xi, f. 4. Placunanomia ione, Gray, P.Z.S., 

 1849 (1850), 123; Hutton, M.N.Z.M., 174; P.L.S. N.S.W.,' ix, 533. 

 Conch. Icon., xi, f. 6 ; C. Tert. M.B.M., 303. Anomia Stowei, Hutton, 

 C.M.M., 83; M.N.Z.M., 173; P.L.S. N.S.W., ix, 532. Placunanomia 

 Stowei, Hutton, J. de Conch., 1878, 55. 



Shell variable in form, mostly orbicular: left valve fairly solid, 

 convex ; the right thin, translucent, flat ; young shells are sometimes 

 quite smooth, but generally they have wavy radiate riblets composed 

 of semicylindrical hollow spines, which form continuous ribs in the 

 adult ; inequivalve and more or less inequilateral. Beak of left valve 

 small, convex, smooth ; dorsal margin sometimes straight, but more 

 often rounded. Sculpture, consisting of radiate ribs on the left valve, 

 sometimes rather distant and wavy, especially in young shells, but 

 usually close together, all intermediate grades occurring ; concentric 

 laminae, partly interrupting the radial sculpture, are more or less pro- 

 minent ; right valve with concentric ridges and grooves, frequently 

 crossed by narrow wavy and interrupted grooves. Colour white, 

 often greenish at the centre. Interior of left valve dark green in the 

 middle, lighter towards the margin ; right valve with a broad white 

 porcellanous area round the foramen, light olive outside. Margins 

 thin, laminated. Ligament in the umbonal cavity, transversely tri- 

 angular. Byssal scar at the upper third of the height in the median 

 line of the left valve, large, oval, radially finely striated ; slightly 

 below and behind, and confluent with it, is the much smaller round or 

 slightly oval adductor-scar, both included in a triangular area. Right 

 valve with a moderately large pyriform foramen, closed above ; on the 



