<»19 PALAEONTOLOGY OF 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 Oloau in 

 Allegany county. 



Geological formation and iocalities. In the Chemung group at Ithaca, and at 

 Bald hill south of Ithaca, Tompkins county; at Philipsburgh near Angelica, and 

 at Rockville in AUcganj- county; near Paintodpost on the Chcnunig river; at 

 Troupsburgh in Steuben county, neai* Olcan in Allegany county, and at liaudolph 

 iu Cattai-augus county. 



Spirifera uiesastrialis. 



PLATE XL. 

 Dtlthyru mesattrialu : 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. 



Ventral 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. 



DoKSAL valve more 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- 

 riciilate, or prolonged into mucronate points. The mesial fold becomes 

 rapidly expanded in its lower part, with the summit broadly rounded. 



Surface clicate with about sixteen to twenty rounded ribs, which are 

 sometimes low and nearly flat, or become more strongly elevated in 

 Bome 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 



