228 Proceedings of the Royal Society of Victoria. 



such organisms as polyzoa, and on that account in all the speci- 

 mens yet to hand the shape is fairly constant; but should wholly 

 attached valves occur, as is not unlikely, the shape would then 

 necessarily he greatly dependent upon the surface of attach- 

 ment. 



Pinna coruata, sp. nov. 

 (Plate XII, figs, -i, .3.) 



Shell thin, triangular, elongate ; valves very convex ; dorsal 

 half bearing about ten smooth longitudinal ribs increasing in 

 breadth posteriorly, with shallow interspaces which also become 

 broader posteriorly, but much mcjre rapidly than the ribs ; dorsal 

 slope abrupt apically, becomes more gradual posteriorly, ulti- 

 mately similar to the ventral slope ; ventral half with numerous 

 close-set concentric lines of growth, and broad well defined 

 undulations parallel to the lines of growth becoming obsolete 

 l)efore reaching the dorsal ril)S. In juxtaposition to the well 

 defined dorsal ribs, and on the ventral slope there are four or 

 ti\e very faintly developed close and narrow longitudinal ribs 

 becoming slightly stronger posteriorly. Dorsal margin at first 

 straight, then rapidly ascending, giving it a distinctly concave 

 aspect ; ventral margin concave about the byssal orifice, then 

 rapidly ecurved becoming regularly convex to the posterior eiid ; 

 posterior margin incomplete, apparently, from the aspect of the 

 shell, gently rounded from the ventral margin. 



Dimensions. — Length of dorsal margin (incomplete), 110 nmi.; 

 width, 55 mm.; greatest thickness thi'ough both valves, 39 nmi. 



Locality. — Eocene sandy limestones, Barwon River, near its 

 junction with the Native Hut Creek. One example collected by 

 31 r. J. Betheras. 



Observations. — This species seems to he closest related to the 

 South Australian ^Miocene species, P. seniicostata, Tate, from th© 

 oyster beds of Adelaide and Aldinga Bay, but as far as I have 

 been able to make out from Professor Tate's description and 

 figure, the i:>resent species is a relatively narrower and more 

 convex form, with a much more abrupt dorsal slope apically, and 

 is without scales on the lont>itudinal ribs. 



