88 [Senate 



RHYNCHONELLA ORBICULARIS ( n. s.)- 

 Sheljl circular, ventricose. Dorsal valve subhemispherical; mesial elevation ex- 

 tending from near the beak to the base, and gradually expanding : beak small, 

 compressed, not rising above the general outline of the shell. Surface marked by 

 about twenty-four strong, rounded, highly elevated plications ; four occupying 

 the middle of the valve, more prominent, forming the mesial elevation; the 

 plications on the lateral cardinal slopes slightly arched at the outer extremities. 

 In some parts of the specimen are indications of regular concentric striae. 

 This description is drawn from a single dorsal valve, but so characteristic as to 

 leave no doubt of the specific distinction. The circular form, full rounded cardinal 

 slopes, the four strong plications of the mesial fold and recurved plications of the 

 sides, are decided characters. 



Geological formation and locality. In rocks of the Chemung group : Steuben 

 county, N.York. 



TEREBRATULA LINCKL.ENI ( n.s.). 



Atrypa : Geol. Report 4th Dist. New-York, p. 202. 



Shell oval, ovate or subelliptical, generally broadest below the middle, varying 

 from moderately convex to very gibbous, sometimes subcylindrical; cardinal 

 slopes of the ventral valve somewhat angular. Ventral valve usually the most 

 gibbous, somewhat flattened in the middle and toward the anterior margin, and 

 sometimes marked by a faint narrow mesial depression. Dorsal valve with umbo 

 narrow and prominent. Surface smooth, or marked by a few concentric lines of 

 growth : these are sometimes strong, and cause an abrupt thickening of the 

 shell near the anterior margin. Shell-structure distinctly and beautifully punc- 

 tate. 

 Geological formation and focaZi^y.InUmestoneof the Hamilton group: Delphi, 



N.York. 



TEREBRATULA RECTIROSTRA ( n.s.). 



Shell elongato-ovate, subovate or subelliptical, the greatest width anterior to the 

 middle. Valves subequally convex, somewhat truncate at base : beak of ventral 

 valve strong, little incurved ; deltidial plates long and narrow; shell abruptly 

 inflected along the umbonial slopes : beak of the dorsal valve incurved, passing 

 within the cavity of the opposite valve; umbo prominent. Surface marked by 

 strong concentric lines of growth. Shell-structure distinctly punctate. 

 This species differs from the last in being less ventricose and arcuate, with beak 



longer and more incurved : it is also larger, and the base truncate. 



Geological formation and locality. Shales of the Hamilton group : Western 

 New-York, 



