ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 81 



INDIANA CURTA, new species 



Plate 5, Figure 9 



This species is founded on a single left valve so much like /. jaba 

 that a detailed description may be dispensed with. However, in 

 looking over the material of /. jaba, which consists of more than 30 

 specimens, this valve was at once separated on account of its com- 

 paratively short form. The difference in this respect is clearly shown 

 by the illustrations of the two forms on Plate 5. /. curta is probably 

 merely a well marked variety of /. Jaha, but in the absence of satis- 

 factory intermediate stages, it has been thought preferable to view 

 it provisionally as a distinct species. 



Dimensions: Length 4.25 mm., height 3.51 mm. 



Occurrence. — Bright Angel shale, just above massive lower sand- 

 stone, near mouth of Bass Caynon, Grand Canyon of the Colorado, 

 Ariz. 



Holotype.—Csit. No. 56466, U.S.N.M. 



INDIANA IMPRESSA, new species 



Plate 10, Figure 2 



Description. — Carapace of medium size, obliquely subovate, the 

 ends nearly equal in width; dorsal edge straight, nearly three-fourths 

 the length of the valves; cardinal angles readily determinable but not 

 sharply defined, apparently more obtuse in the left than in the right 

 valve; anterior margin with greatest prominence and curvature in the 

 upper half, trending backward in the lower half; posterior margin 

 strongly rounded in the middle and lower thirds, occasionally, as in 

 the figured specimen, slightly truncate in the upper third; ventral 

 margin gently convex, its course in general approximately paralleling 

 the hinge. Surface of valves on the whole gently convex, broadly 

 depressed in the median third, sloping rapidly at the ends which are 

 blunt; an ill-defined furrow within the ventral edge forms a rather 

 wide, flattened border that dies out before reaching the ends. Test 

 white or brown, smooth, apparently not punctate. 



Dimensions: Length 4.1 mm., height 2.7 mm., greatest thickness 

 about 1.0 mm. 



Remarks. — This species is readily distinguished from all others 

 referred to the genus by the broad median depression of the surface. 

 Of known forms it is perhaps nearest /. faba var. intermedia, but as 

 this also is without the median depression and differs further in 

 minor details of outline, and in the more important fact that it has 

 no furrow in the ventral slope, there is little excuse for confusing 

 them. 



In its most characteristic feature /. impressa suggests Dielymella 

 dorsalis, which also occurs in the Bright Angel shale. However, 

 2607—31 6 



