143 



straight; valves closed, inner margin crenulated, conspicuously 

 so on tlie posterior side. Surface ornamented by flatly rounded 

 radial ribs, broader than the interspaces, and concentric striae 

 and folds, producing granular thickenings at the intersections ; 

 the radial ribs are very prominent on the posterior slope. 

 Hinge area very narrow, teeth numerous, transverse, becoming 

 larger and slightly oblique towards the extremities. 



Dimensions. — Length, 9 ; width, 5 ; thickness through both 

 valves, four millimetres. 



Locality. — Not uncommon in the upper beds at Muddy 

 Creek, Hamilton. 



Macrodon Cainozoicus, spec. nov. PI. x., fig. 4. 



Shell elongate, subrhomboidal, somewhat compressed, inequi- 

 lateral ; anterior side short, acutely angular, somewhat 

 acuminate ; posterior side elongated, truncated with a well 

 defined posterior slope ; hinge line straight ; ventral margin 

 insinuated in the middle, the antero-ventral line rapidly as- 

 cending. TJmbones subanterior, not widely separated, slightly 

 incurved. Surface with concentric flat ridges, broader than 

 the deep sulci, and radial threads interrupted by the furrows. 

 Internal margins of valves smooth ; ligamental area narrow, 

 with a few longitudinal ridges ; teeth radiating from an eden- 

 tulous centre, five or six anterior, short, oblique ; posterior 

 several, laminar, parallel with the hinge line. 



Dimensions of a large specimen : — Length, 27 ; width, 10 ; 

 thickness through both valves, 9 millimetres. 



Localities. — South Australia : calciferous sandstones, Eiver 

 Murray Cliffs, near Morgan ; Adelaide bore ; Aldinga. Vic- 

 toria ; Schnapper Point, Muddy Creek and Corio Bay {B.T.). 



Prof. Hutton, in Trans. Is.Z. Institute, vol. xvii., p. 831, 

 1885, describes an a.rk under the name of j\Iacrodon australis, 

 which I think, after examination of an authentic specimen, 

 would be better placed in the genus Ciicullcsa. My species 

 does not conform with the typical species of the genus, but it 

 is generically identical with certain Jurassic and Cretaceous 

 forms which have been associated therewith. 



Gi-ENTJS CtJCTILL^A. 

 SYNOPSIS OF SPECIES. 



Very gibbous, with granulated ridges. C. Corioensis. 



!N"ot so gibbous ; umbones narrower, ribs more numerous, 



narrower, and finer. C. Adelaidensis, 



