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PALAEONTOLOGY OF NEW-YORK. 
Spirifera formosa. 
PLATE XXVIII. 
Spirifer formosa : Hall, 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 strise, which often become dis¬ 
tinctly imbricate and stronger towards the margin. In well-preserved 
specimens, remains of extremely fine radiating striae 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 age of the Hamilton group 1 
Bake-oven, Illinois. 
