171 



Dimensions. — Anteroposterior diameter, 42 ; urnbo-ventral 

 diameter, 29*5 ; sectional diameter o£ united valves, 20 milli- 

 metres. 



Localify. — Common in the upper beds at Muddy Creek. 



This species has the aspect of M. rufescens, Lamarck, a well- 

 known shell inhabiting the temperate regions of Australia, from 

 which it differs by being more attenuated in front and behind, 

 proportionately longer, and having finer ornament. 



Mactra Hamiltonensis, spec. nov. Plate xvii., figs. 4a — 46. 



Ovately elongate, subequilateral, slightly convex, thin,, 

 shining. The anterior side is rather shorter than the pos- 

 terior, the former is broader and obtusely rounded at the end, the 

 latter is narrowed towards the end, where it is sharply rounded. 



The ornament consists of thin elevated growth lines, which 

 are here and there confluent one with another. 



The pallial sinus is sharply rounded at the end, and reaches 

 a third of the length across the valves. 



Dimensions. — Antero-posterior diameter, 23 ; umbo-ventral 

 diameter, 18 ; sectional diameter of united valves, 9 milli- 

 metres. 



Locality. — Common in the upper beds at Muddy Creek. 



This species is the analogue of the living 31. polita, Chemnitz r 

 of Southern Australia, from which it differs by its flatter valves 

 and less triangular outline, being proportionately longer and 

 the ventral margin not so broadly curved. 



Mactra Howchiniana, spec. nov. Plate xvii., figs. 3a— 35. 



Oblong, attenuated at both extremities, inequilateral, thin, 

 shining. The anterior side is shorter and narrower than the 

 posterior, the front-dorsal margin is slightly concave, the post- 

 dorsal slightly arched ; the ventral margin is almost straight 

 medially, but ascends more rapidly behind than in front. 



The surface is concentrically finely ridged and striated. 



The lateral teeth are transversely wrinkled on the dorsal 

 face. The pallial sinus is rather broad, rounded at the apex, 

 confounded with the pallial line, and extends about half-way 

 across the valve. 



Dimensions. — Antero-posterior diameter, 41 ; umbo-ventral 

 diameter, 23 ; sectional diameter of both valves, 12 millimetres. 



Locality. — Eare in the lower beds at Muddy Creek. Young 

 shells not uncommon in the calciferous sand-rock of the Eiver 

 Murray Cliffs, near Morgan ; rare in the clays at Schnapper 

 Point, Hobson Bay. 



The specific name is in compliment to Mr. Walter Howchin, 

 F.G-.S., who has elaborated for the author the Foraminifera of 

 the Older Tertiary beds of Australia. 



