•3 12 
PALAEONTOLOGY OP NEW-YORK. 
localities. The species has been collected in the shaly beds in the higher part of 
the group near the southern limits of Steuben county, and it occurs in a coarse 
sandstone and conglomerate in the upper part of the formation near Olean in 
Allegany county. 
Geological formation and localities. In the Chemung group at Ithaca, and at 
Bald hill south of Ithaca, Tompkins county; at Philipsburgh near Angelica, and 
at Rockville in Allegany county; near Paintedpost on the Chemung river; at 
Troupsburgh in Steuben county, near Olean in Allegany county, and at Randolph 
in Cattaraugus county. 
Spirifera mesastrialis. 
PLATE XL. 
Delthyris mesastrialis : Hall. Geol. Report Fourth District, p.269, f. 1. 1843. 
Shell large, semielliptical, more or less gibbous, with the cardinal angles 
rounded or produced into mucronate extensions : surface plicated. 
Yentral valve variable in its convexity, being gibbous or subventricose 
in the upper part, curving gently to the front and baso-lateral margins, 
while it is depressed or nearly flat towards the cardinal extremities. 
Beak incurved over the area, which is of medium height and continued 
to the cardinal extremities : mesial sinus rapidly expanding and be¬ 
coming very wide below, in some specimens being of moderate depth 
and in others very deep, and rounded in the bottom, with the margins 
well defined. 
Dorsal valve moi;e or less gibbous, often with a very regular convexity 
when the cardinal extremities are not extended. In other examples 
the central portion is somewhat regularly convex, and depressed or 
flattened towards the extremities, which may be rounded, slightly au- 
riculate, or prolonged into mucronate points. The mesial fold becomes 
rapidly expanded in its lower part, with the summit broadly rounded. 
Surface plicate with about sixteen to twenty rounded ribs, which are 
sometimes low and nearly flat, or become more strongly elevated, in 
some of the larger individuals. There are usually from eight to twelve 
ribs on each side, which are conspicuous, while the remainder are at¬ 
tenuate and obscure. The plications, together with the mesial fold and 
