102 



and Adchiide. Muddy Creek, Hcunilton, a few small flat 

 smooth examples {J. Dennant). 



It is not with eoiifidence that I refer the fossil under review- 

 to the recent species inhabiting the waters of the New Zealand 

 and South Australian shores. It exhibits, however, very great 

 variation, especially in respect to the radial ornamentation. 

 X'sually the rays are obsolete, except towards the margin. In 

 other examples the rays are moderately strong and numerous, 

 as in recent examples, but occasionally the ribs are thickened, 

 elevated, and raised into sub-spiny scales, the ribs being gene- 

 rally few in number. 



Though the attached valve is unknown, despite the fact that 

 several score of the free valves have been collected, yet there 

 cannot be a doubt that it is correctly placed in the genus 

 FJacunanomia, as the shelly base of the plugs are conspicuous 

 objects on oysters and other molluscous shells associated in 

 the same deposit. 



Placunanomia sella, spec. nov. PL v., figs, la— Ic. 



Shell suborbicular to triangular-ovate, thin, inequivalve, in- 

 equilateral ; lunbos depressed, marginal. Left valve convex 

 from the ventral to the dorsal margin, w^ith lateral areas more- 

 or less elevated ; right valve concave, with depressed sides, 

 sinus small pyriform. Surface of both valves ornamented with 

 numerous radial threads and lamellae of growth. In young 

 shells the radii are minutel}- scabrous; a variable character 

 consists of radial and concentric undulations. Interior of 

 left valve with a submarginal oblique cartilage pit and two 

 oval subcentral muscular impressions ; the latter are super- 

 imposed, that produced by the muscle of the plug the larger. 

 Interior of right valve with a single adductor impression 

 within the posterior margin of the disc, posterior lobe of 

 notch overla])ping on the anterior lobe, and more or less 

 agglutinated therewith ; the margin of the notch with a raised 

 border. In adult specimens the marginal border is excessively 

 developed to form a tubular sheath, to which the shelly base of 

 the plug becomes adherent. 



Dimensions of a largish specimen. — Length, 43 ; height, 40^ 

 millimetres. 



LocaJifies. — South Australia : Abundant in the basal part of 

 the calciferous sandstone of the Eiver Murray cliffs at 

 MacBean's Pound, near Elanchetown ; in a well-sinkin<i\ 

 " JS'ine-Mile Camp," near the Xorth-AVest Bend {B.T.) 



TasDiania : Table Cape, determined from a left valve re- 

 ceived from Mr. E. M. Johnston. 



"S'ictoria : A thin, translucent form, with flattish sides, is 

 common at Muddy Creek ; a very small example, Schnapper 

 Point, Hobson's Bay {B.T.) 



