PALAEONTOLOGY OP NEW-YOEK. 
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-tliird of its length from the apex. 
The characteristic features and expression of the fossil are shown in the ac¬ 
companying woodcut. 
SflBIFERA MACRONOTA. 
Geological formation and localities. This species occurs on the shore of Seneca 
lake; at Bristol, Ontario county; at Geneseo and Moscow in Livingston county; 
at Darien, Genesee county, and at Hamburgh and Eighteen-mile creek in Erie 
county. A 
The three preceding species (Spirifera medialis, S. angusta and S. ma- 
cronota ), in what may be termed typical specimens, are so clearly dis¬ 
tinguishable one from the other, that we 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 
frpm 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. 
