264 



PJiLJlONTOLOGY OF NEW-YORK. 



Position and locality. This species occurs in the limestone of the Niagara group, in its 

 eastern extension into Oneida county. The only place I have been able to find it, is at the 

 locality cited by Mr. Vanuxem, on the banks of the creek a short distance west of Vernon 

 centre. At this place the shells appear upon the surface of the thin layers of limestone, and 

 I have been unable to obtain any perfect specimens. 



625. 8. SPIRIFER NIAGARENSIS. 



PL. LIV. Fig. 5 a- t. 



Delthyris niagarensis. Conrad, Jour. Acad. Nat. Science, 1842, Vol. viii, p. 261. 

 — — Hall, Geol. Rep. 4th District, 1843, pag. 105, fig. 1. 



Semioval or semicircular, convex or moderately gibbous in the centre, and compressed to- 

 wards the sides ; valves nearly equally convex ; dorsal valve having the beak elevated and 

 incurved over the area ; area medium width ; hinge-line usually shorter than (rarely extended 

 beyond) the width of the shell, with the extremities rounded. Surface marked by twenty to 

 thirty rounded, depressed plications (in young shells not more than ten or twelve), longi- 

 tudinally striated by fine equal striae, which are equally conspicuous on the mesial sinus and 

 elevation. 



This shell is typical of the Niagara group, and is always readily recognized by its rounded 

 plications, which are evenly striated in a longitudinal direction. There is usually a stronger 

 line in the depression between the plications. The surface is similarly striated to the margins, 

 but the plications gradually die out towards the cardinal extremities. The area in perfect shells 

 is of medium width, but usually appears as if very narrow, and the valves nearly closed, from 

 compression. From its large size, it is rarely found in a good state of preservation, and the 

 valves are often distorted and compressed, giving an outline very different from the natural 

 form. Although having a wide horizontal range, its vertical range is extremely limited, oc- 

 curring in considerable numbers only through a very limited thickness of the shale, and rarely 

 in the limestone above. 



Fig. 5 a, b. Ventral and dorsal views of a young specimen, which has only about five or six 



defined plications on each side of the mesial fold and sinus. 

 Fig. 5 c. Front view of the same. 



Fig. 5 d. Profile view of another more gibbous specimen. 

 Fig. 5 e, f, g. Ventral, front and profile views of a well preserved specimen, which has about 



eight defined plications on each side of the mesial sinus and fold. 

 Fig. 5 h, i. Ventral and profile views of a well preserved specimen, of the ordinary size of this 



species. 

 Fig. 5 k. A flattened dorsal valve of about the same size as the preceding specimen, showing 



the change in form produced by compression. 

 Fig. 5 I. A flattened ventral valve of a specimen somewhat larger than the preceding. This 



condition of specimens is a very common one in the shale : indeed the perfect form 



is the exception. 



