Orthis. 



528 



Spec. 11,745 (000) Hamilton upper shale ; Jericho school 

 house, 12,020, ditto', Crawley hill, 12,460, ditto,— In Columbia 

 Co. Hemlock, Fishing Creek Section, in a richly fossiliferous 

 bed 100' beneath the top of the Hamilton. G7, p. 75, 229.— 

 In the Oil region, CarlPs collections, Upper Chemung (C. E. 

 Hall, MS. Rt. Dec. 30, 1876.)— F///^; ( Vlll-lXf) 



Orthis pennsylvanica, new species, Simpson, Trans. Am. 

 sm y-^rv^ fv ..^ ^ 'e\i\\o%, Soc. 1889, Vol. 



sentially circular, in 



nearly all the specimens 



S''^:''liy.:i^^^^ teSSfelsl^^^ observed, the height and 

 v/i'.'//h. t!Ai..jh!^^^m m;:i^mi--mam^^ Width being equal ; hinge 



line short, length equal 

 APS) ^f.i889> to half the width of the 



shell ; cardinal extremities rounded ; lateral and basal mar- 

 gins regularly rounded, except in the middle basal margin of 

 the ventral valve, where there is a slight constriction. Dorsal 

 valve somewhat gibbous, greatest convexity a little above the 

 middle; rapidly curving to the cardinal and lateral margins ; 

 a little more gradually to the basal margins. Along the mid- 

 dle of the valve is a flattened or slightly depressed area, nar- 

 row at the beak, gradually growing wider, and comparatively 

 broad at the base. Ventral valve. A perfect specimen has 

 not been observed, but gutta-percha casts have been taken 

 from impressions of fragments in the rock which probably be- 

 longed to this specie^. 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. Surface marked by prominent, subangular, radiating 

 striae, increasing by bifurcation, of uniform size at the margins, 

 where there are 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 striae, which on many 

 specimens are obsolete ; also, at irregular intervals, by lines or 

 varices of growth. On the cast of the dorsal 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 depression along the middle. On the ventral 



