CLFNTON, LOWER HELDEKBERG, CHEMUNG, AND WAVERLY GROUPS. 4-43 



The form of this species is very simihir to that oi Meristella hella, of the Lower 

 Helderberg group, but that species has a depression on both the ventral and dorsal 

 valves, while this species has a fold on the dorsal valve. The subrhomboidal form 

 distinguishes it from any species of the Upper Helderberg gi-oups. 



formation and locality/. Chemung group, near Warren, Warren county, Penn- 

 sylvania. 



RnYNCHONELLA (STENOCIIISMA) L.EVIS Simpson, n. sp., Fig. 8. 



Shell ovate or subtrigonal ; valves subequally convex in young shells ; in 

 older shells the dorsal valve usually the most gibbous. The width is slightly less 

 than the height ; greatest width about two-thirds the length from the 

 beak ; raai'gins from the apex to this point slightly curved outward, 

 nearly straight, then somewhat abruptly rounding, and at the base pi'o- 

 duced in a slight extension. 



Ventral valve depressed convex, slightly gibbous at about one-third ^■''■^- ''''■'^^■ 

 the length from the apex, curving abruptly to the cardinal and antero-basal margins, 

 more gradually to the lower lateral margins, becoming depressed in a shallow sinus, 

 which commences at about one-third to one-half the length from the beiik; beak 

 slightly inciu'ved, strongly projecting beyond the beak of the opposite valve. 



Dorsal valve convex, greatest convexity at or a little below the middle, gradually 

 curving to the apex and cardinal margins, more abruptly to the lateral margins ; 

 mesial fold commencing at a little above the middle and often becoming prominent at 

 the front; beak small and nearly straight. 



Surface marked by from ten to twelve rounded or subangular plications, wliicii 

 become obsolete on the upper portions of the valves. The mesial fold is composed of 

 three plications much more piominent than the others. In the sinus there are three 

 plications smaller tliiiii the others; the two plications bordering the sinus larger ilum 

 those on other portimis of the shell. 



The specimens observed have a length of 9 or 10 mm., and a widlli of IVoni 7 to 

 9 mm; a transverse section broadly oval, having a width of 5 or G mm. 



This species is easily distinguished from any other of this formation at present 

 known. The outline is very similar to some of the forms of Ji. (Slenocliisma) exhnin, 

 of the Chemung group, but the shell is smaller, the plications of the foUl and siiuis 

 are fewer and the plications become obsolete on the upper half of the valve. 



Formation and locnUty. Clinton group, limestone, two miles south-west of litll's 

 mills, Blair county, Pennsylvania. 



