the PALEONTOLOGY OF NEW- YORK. 



GEKI'S NpIRIFERA (Sowerby). 



The Genus Spirifera, established by Sowerby in 1815, embraces a very 

 natural group of shells, which are for the most part very readily distin- 

 guished by their external form and characters. The presence of internal 

 spires is tiot alone sufficient to distinguish them, since these organs are 

 common to other very distinct genera. 



The prevailing and characteristic form of the shells of this genus is 

 somewhat trigonal ; but they vary to subelliptical, ovoid and subcircular 

 in outline. The hinge-line may be shorter or longer than the greatest 

 width of the shell, and the cardinal extremities are sometimes obtusely 

 rounded, and often produced into wing-shaped extensions which termi- 

 nate in acute points. The surface is smooth, or marked by radiating 

 costa3, presenting a great variety of aspect and ornament ; and the centre 

 of the valves is usually marked by an elevated fold on the one valve, 

 with a cprresponding sinus in the other : this feature, however, is 

 more or less obsolete in some of the species. The shell-structure is 

 fibrous. 



The valves articulate by means of teeth and sockets. The beak of the 

 ventral or larger valve is more or less elevated above the other, and may 

 be straight or recurved. The area or space beneath the beak may be nar- 

 row or large, flat or concave, or inclined forwards. This area is divi- 

 ded by a triangular fissure, which is often more or less closed in its 

 upper part by a pseudo-deltidium. In the dorsal valve the area is narrow, 

 usually linear, with a wide fissure, which is partially filled by the car- 

 dinal process. 



In the ventral .valve the triangular fissure is' bounded by vertical 

 shelly plates, which extend from the beak to the bottom of the valve, 

 and support, on ejich side at the base of the fissure, a short and usually 

 strong hinge-tooth. These fissure-walls are usually short diverging plates; 

 but they are sometimes nearly parallel throughout their length, and reach 

 to the middle of the valve. They sometimes converge so as nearly to 



