999 PALEONTOLOGY OF NEW-YORK. 



growth, which give an irregular margin to the area, and are often 

 crowded near the front of the shell, causing an abrupt truncation of 

 the extremities of the ventral area. 

 Interior of the ventral valve with comparatively small but well-marked 

 muscular impressions ; that of the dorsal valve not satisfactorily deter- 

 mined. The margins of the interiors of the valves are usually crenu- 

 lated by the plications and the stronger concentric lamellae of growth. 

 The fissure is sometimes partially covered by a pseudo-deltidium, and 

 closed by a solid septum for one-third of its length from the apex. 

 The characteristic features aud expression of the fossil are sbo^-n iu the ac- 

 companying woodcut. 



SrialFEBA KACBONOTA. 



Geological formation and localities. This species occurs on the shore of Seneca 

 lake; at Bristol, Ontario county; at Gencseo and Moscow in Livingston county; 

 at Darien, Genesee county, and at Hamburgh and Eighteen-mile creek in Erie 

 county. 



# 



The three preceding species (Spirt/era medialis, S. angusta and S. ma- 

 cronota), in what may be termed typical specimens, are so clearly dis- 

 tinguishable one from the other, that Ave have no difficulty in designating 

 them. It is only in large collections of specimens that we find individuals 

 which are not readily referable to either of them. The young of S. me- 

 dialis are often much extended at the cardinal extremities ; and though 

 generally distinguishable from the young of S. macronota by the lower and 

 more arcuate area, yet there are gradations which often seem to unite the 

 two. A similar difficulty occurs when we attempt to separate S. angusta 

 from the young of S. macronota ; and there are numerous individuals, not 

 always, it is true, in a perfect state of preservation, which it is nearly 

 or quite impossible to refer with satisfaction to either of the species. 



