( LINTOX, LOWEU IIELUEUIJEUG, ClIEMUXG, AND WAVKKLV GUOUI'S. -A-')! 



OKTIIIS PENNSYLVANICA Simpson, n. sp., Fig. 1. 



Shell essentially circular, in nearly all the specimens observed, the height and 

 width being equal ; hinge line short, length equal to half the width of the shell ; car- 

 dinal extremities rounded ; lateral and basal mar- yms 

 gins regularly rounded, except in the middle basal 

 margin of the ventral valve, where there is a slight 



/>^V<. 



constriction. 



Dorsal valw .somewhat gil)l)()us, greatest con- 'jsT ^^^y^,^ 



vexity a little above the middle; ra[)idly curv- a^s- ttis-3'' 



ing to the cai'dina! and lateral margins; a little more gradually to the basal margins. 

 Along the middle of the valve is a flattened or slightly depressed area, narrow at the 

 beak, gradually growing wider, and compai-atively broad at the base. 



Ventral valve. A perfect specimen has not been observed, but gutta-peicha casts 

 have been taken from impressions of fragments in the rock which probably belonged 

 to this species. The valve is flattened, or of much less convexity than the dorsal 

 valve, with a slight elevation along the middle, corresponding to the depression of the 

 opposite valve. 



Siuface marked by piomincnt, subangular, radiating stria?, increasing by bifur- 

 cation, of uniform size at the margins, where there arc twelve in the space of 5 mm. ; 

 a .short distance below the beak there are twenty in the same space. The radii are 

 crossed by fine indistinct concentric strife, which on many specimens are ob.solete ; 

 also, at irregular intervals, by lines or varices of growth. 



On the cast of the dor.sal valve the distance from the beak to the lower margin 

 of the muscular impression is about one-half the length of the valve. Impression ovate 

 in outline ; width equal to or very slightly less than the length, with a deep depies- 

 sion along the middle. On the ventral valve the impression is ovate, bilobed. The 

 margins are distinctly lobed in four divisions. The greater portion of the speciuiuns 

 observed have a diameter of Irom 20 to 25 mm. 



This species may be distinguished from Orthis impressa, of this formation, by its 

 smaller size and its more circular outline; from Ortliis tior/n, by its more circular out- 

 line, less conspicuous sinus and elevation, and ovate muscular impression. 



Formation and localities. Chemung gi"oup, Sullivan township, Tioga county ; P. 

 and E. Railroad, between Ludlow and Wetmore, and Kin/ua creek near west line nf 

 McKcan county; and at various other localities in Pennsylvania. 



ORTIILS SUBCIRCaLA Simpson, n..'=p., Fig. 2. 

 Shill liioadly oval, nearly circulai- ; height usually about three-fourths the width : 

 gi'catest width just below the middle; cardinal line shoil, length less than lialf the 



