9«0 PALEONTOLOGY OF NEW-YORK. 



Spiiifera formosa. 



PLATE XXVin. 

 Spirifer formoia ; IIall, Tenth Report on the State Cabinet, p. 154. 



Shell semielliptical or subquadrate, more or less ventricose : v. ives 

 about equally gibbous ; hinge-line equal to the greatest width of the 

 shell, with the extremities sometimes a little salient. Surface plicate. 



Ventral valve most prominent above the middle and towards the umbo, 

 with the sides regularly curving to the margin ; the beak small, and 

 neatly incurved over the area. Area rather high, continuing to the 

 cardinal extremities, concave, and striated longitudinally ; fissure 

 large, wider than high : mesial sinus shallow, of moderate width, and 

 sometimes with an obscure depression along the centre. 



Dorsal valve regularly convex, a little flattened or sometimes concave 

 towards the cardinal extremities : mesial fold but little elevated, flat- 

 tened upon the summit, and sometimes a little depressed in the middle 

 and continuing distinctly to the neatly incurved beak. Area distinct 

 of moderate height, and nearly parallel with the longitudinal axis of 

 the shell. 



The surface is marked by from fifteen to seventeen slender rounded 

 plications on each side of the mesial fold and sinus ; and these are 

 crossed by fine concentric undulating striae, which often become dis- 

 tinctly imbricate and stronger towards the margin. In well-preserved 

 specimens, remains of extremely fine radiating strife are found. 



This is a neat symmetrical species, with a gracefully curved outline and salient 

 angles. In profile, the beak of the ventral valve projects but little beyond that of 

 the dorsal valve. The mesial fold, which is usually flat in the middle and lower 

 part of the shell, is often distinctly grooved along the centre in the upper part, 

 and this depression sometimes continues to the base. 



Geological position and locality. In limestone of the ago of the Hamilton groups 

 Bake-ovcn, Illinois. 



