376 



Shell quadrately-orbicular, tliin, translucent, inequilateral, 

 equivalve ; posterior broad, with subtruncated margin ; anterior 

 narrower and rounded ; surface has rather coarse concentric 

 growth-lines ; umbones small, acute. The author compares it with 

 D. tasmanica, Tenison-Woods, from which it differs by its quadrate 

 outline and regular sculpture, and in being less inequilateral. 

 D. zealandica, Gray, has a rounded ventral margin and is more 

 inflated, but otherwise resembles the present species. 



Dimensions. — Height 22 mm.; length 23"5 mm. 



Form, and Loc. — Eocene : Muddy Creek, Victoria. 



L. 4805. Left valve of the neanic stage of growth. 



Presented ly John Dennant, Esq. 



L. 9854. Examples of both valves. Purchased. 



Diplodonta sub orbicularis, Tate (sp.). 



1887. Sacchia suborbicularis, Tate, Trans. Eoy. Soc. South Aust. vol. ix. 



p. 147, pi. xviii. figs. lOa-c. 

 1894. Mysia [Felania) suhorbicularis, Tate, Journ. Roy. Soc. N.S.W. 



vol. xxvii. p. 187. 



Shell differs in shape on comparison with D. sulquadrata, being 

 triangularly (not quadrately) orbicular, depressed, posterior side 

 rounded, somewhat produced anteriorly; umbones smooth, remainder 

 of surface marked with distant growth-folds ; left valve with two 

 divergent cardinal teeth ; in the right valve the two cardinal teeth 

 are separated by a triangular cardinal pit ; no lateral teeth ; 

 cartilage pit broad ; pallial line entire. 



Dimensions. — Height 8 mm. ; length 7'5 mm. 



Form, and Loc. — Eocene : Muddy Creek, Victoria. 



L. 9856. Specimens of both valves of the adult. Purchased. 



Family DOISTACID^. 



Genus DONAX, Linnaeus. 



[Syst. Nat. ed. 10, 1758, p. 682.] 



Shell inequilateral, trigonal, oval, or cuneiform ; umbones 

 directed backwards (as iu Nucula, etc.); posterior side much 

 shorter than the anterior ; margin usually creiiulated ; right valve 



