lOG HORN EXPKDITION — PALAEONTOLOGY. 



S/>. char. — Oval-trigonal, inequilateral, umbones anterior and directed 

 forwards ; moderately convex, posterior margin roundly truncated, ventral margin 

 nearly straight, anterior margin shortly rounded, post-dorsal region depressed. 

 Hinge-line steeply inclined and straight posteriorly ; less declinous and slightly 

 curved anteriorly ; teeth numerous, small, regular and transverse, thirty on the 

 posterior side and ten on the anterior side. Surface of shell marked hy strong- 

 folds of growth and coincident strise. Diameters, uinbo-ventral, and antero- 

 posterior, 20 mm.; sectional, 11 mm. Mr. Etheridge's figures represent a more 

 oblong shell than my type, and connects with a still more oblong variety, but the 

 trigonal form is the more common. 



Loc. — In limestone. Middle Valley at Tempe Downs, Petermann Creek, by 

 camp at Laurie's Creek, north of Tempe Vale, and Chandler Range. 



Isoapca eastii, sp. nov. (Plate II., Figs. 12rt, 12/'). 



Sp. char. — Shell transversely oval-oblong, very inequilateral, umbones almost 

 terminal; inflated dorsally and medially, somewhat compressed antero-ventrally ; 

 posterior margin roundly truncated, ventral margin nearly direct but rapidly 

 curving upwards to the anterior margin ; hinge-line slightly arched and long on 

 the posterior side, short and almost perpendicular on the anterior side. Hinge- 

 teeth transverse, arched, regularly disposed, increasing in .size from the umbo 

 outwards, about seven on the anterior side and about fifteen on the posterior side. 

 Cast smooth, the anterior adductor scar large and deep. Antero-posterior 

 diameter, 44 mm. ; unibo-ventral diameter, 32 mm. 



Var. modiolsefOPmis (PI. II., fig. 12^) differs by its obliquely cylindrical 

 sliape and more prominent umbos, but is connected with the type by intermediate 

 gradations. The cast presents a few growth-lines and tine coincident stri:t>. 



Loc. — In sandstone and (juartzite underlying the fossiliferous limestone at 

 Middle Valley (Tempe Downs). 



Isoarca corrugata, sp. nov. (Plate I., Fig. 8.) 



Sp. char. — Slicll small, transversely oval-oblong, inequilateral, moderately 

 convex ; umbon(^s ante-median, incurved, approximate. Anterior side short and 

 rounded ; ventral margin slightly ecurved medially ; posterior side about twice as 

 lung as the anterior one, slightly narrowing towai-ds the subtruncate margin ; post 

 dorsal margin straiglit declinous, antero-dorsal margin slightly concave. Hinge 

 line witli direct, transver.se, uninterrupted, close teeth, with narrower interstitial 



