ORTHIS OF THE TULLY LIMESTONE. 
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ORTHIS OF THE TULLY LIMESTONE. 
There is a single species of Orthis, the 0. tulliensis, which is quite cha¬ 
racteristic of the Tully limestone. It is unknown in the Hamilton group 
in New-York, or in any rock below. Though of the type of 0. multistriata 
and 0. propinqua, it is usually readily distinguished from either of these in 
its exterior characters, but more particularly in its muscular and vascular 
impressions. Its geographical distribution is very restricted, so far as 
known. It occurs in Onondaga and probably in Madison county, and in 
Cayuga and Seneca counties; while it is not found on Canandaigua lake, 
or at any point west within the State of New-York. The Orthis iowensis 
of the Hamilton group in Iowa is a very similar and perhaps identical 
species, its differences being possibly due to physical causes and wide 
geographical separation. 
OrtSiis tulliensis. 
PLATE VJI. 
Orthis tulliensis : Vanuxem. Geol. Report Third District New-York, 1843, p. 163. 
Compare Orthis iowensis, Geol. Report of Iowa, Part ii,' p. 488. 
Shell rotund, transverse or nearly subcircular, the length and breadth 
about as seven to eight or eight to nine, the depth varying from two- 
thirds to seven-eighths of the length; hinge-line and area a little less 
than two-thirds the width of the shell. Dorsal valve the larger and 
very gibbous, the greatest gibbosity a little above the middle, sloping 
very abruptly to the beak, and more broadly curving to the front and 
• sides : front sinuate in older specimens ; the area inclined towards the 
- area of the opposite valve, and the beak distinctly incurved, extending 
slightly beyond the limit of the area. Ventral valve somewhat gibbose 
above the middle, becoming gradually depressed, and marked by a 
broad undefined sinus which gives a more or less curved outline to the 
front according to the gibbosity of the shell : area elevated and slight- 
