IlOllX EXPEDITION — PAL.EONTOLOG V. 107 



sufkets, alioui liftucn uii tliu postcrioi' side' and six un the aiituriur side (but the 

 actual niiiuljci' may probably bo ten ; the hinge-line on the anterior side is incom- 

 plete in the only specimen showing this part of the interior). Surface ornamented 

 with aljout eight strong corrugations and nearly a direct post-unibonal ridge. 

 Anteroposterior diameter, G mm. ; umljo-ventral diameter, 1 nnn. 



Loc. — Thickly covering the linu;stone surfaces at Middle Valley (Temjje 

 Downs), also Ilpilla Gorge. 



Isoarca orbicularis, sp. nov. (Plate II., Figs. \^a-b.) 



umi)o 



Sp. liuir. — Shell orljicular, moderately convex, without ornament 

 subspiral, lai-ge, d(^pressed, obtuse, far-distant from the dorsal margin, almost 

 subcentral ; the anterior dorsal margin is arched upwards to become adnate with 

 anterior part of the hinge-area, thus simulating Exogyra or the operculum-like 

 valve of a Reqiiienia. Hinge-teeth occupying a semi-elliptic curve, coincident with 

 the post-dorsal margin of the shell, but receding more and more from the antero- 

 dorsal margin of the shell towards its anterior exti'emity. Teeth numerous and 

 closely-set ; on the extreme posterior side they are curved, thence becoming more 

 and more directly transverse and smaller, and finally almost obsolete on reaching 

 the anterior atlductor scar. Adductor scars inconspicuous. Antero-posterior 

 diameter, 23 nnn. ; transverse diameter, 20 nnn. 



Loc. — Abundant in ijuartzite at Mount Watt. 



Isoarca crassatellaefonnis, sp. nov. (Plate II., Fig. 19.) 



Sp. iliar. — Shell transversely suboval or subtrapezoidal, subine([uilateral ; 

 anterior side rounded ; ventral margin nearly straight, with a moderately deep 

 insinuation post nu'dially ; posterior margin t)blii[uely truncated, the dorsal margin 

 about ecjually sloping on each side. Umboues moderately iiiHated, submedian (a 

 little anterior) ; posterior area defined by a subcarination extending from the umbo 

 to the post-ventral margin. Hinge-line with numerous small, transverse teeth. 

 The surface of the shell, anterior to the posterior carination, is ornamented )jy 

 wide, convex folds (about twelve) separated by linear sulci coincident with the 

 anterior and \entral margins. Inner margin (jf valves edentulous. ^Vntero- 

 post(!rior diameter, IS nnn. ; umbo- ventral diameter, 12 nnn. ; greatest sectional 

 diameter, 9 nnn. 



Loc. — Numerous in (juartzite with /. orbicular'n at Mount Watt. 



