ORTHIS OF THE TULLY LIMESTONE, SS 



ORTIIIS OF THE TULLY LIMESTONE. 



There is a single species of Orthis, the 0. tullimsis, which is quite cha- 

 racteristic of. the Tullj 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. 



Orthis tiilliensis. 



PLATE VIL 



OrthU tullientu : Vanuxem, Geol. Report Third District New-York, 1848, p. 163. 

 Compare Orthu ioirentu, Gcol. 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 ^inus which gives a more or less curved outline to the 

 front according to the gibbosity of the shell : area elevated and slight- 



