No. 109.] 107 



Ortiiis umdonata. 



Orthis jimbonata, Cuxuad : Journal Acad. Nat. Scti. Pliilack'lpliia, Vol. viii, 



1842, pa. 204, ])l.lt, f.21. 



Shell semielliptical in outline, witli a prominent beak, plano- 

 convex : dorsal valve slightly concave below, convex at the 

 umbo : ventral ''■valve profoundly ventricose, with a narrow 

 longitudinal sulcus ;" umbo extremely elevated; beak abruptly 

 incurved; hinge-line greater than the width of the shell below, 

 slightly rounded at the extremities. 

 Geological position a?id locality. In the shales of the Hamilton group : 



at Moscow, Darien, shore of Lake Erie, at Eighteen-mile creek, and 



generally distributed in Western New- York. 



Orthis umhonata. 



Orthis PRiEUMBONA (n.s.). 



Shell plano-convex : dorsal valve slightly convex, much shorter 

 than the opposite, height and width about as three to four, 

 rounded at the cardinal extremities : ventral valve extremely 

 gibbous and highly arcuate; umbo rising much above the hinge- 

 line; beak small, strongly incurved ; mesial portion often flat 

 and sometimes depressed beneath' the umbo. In some specimens, 

 a narrow slightly depressed line extends from near the beak to 

 the base of the shell. Surface marked by fine radiating and 

 concentric lines, which, under a magnifier, give it a finely 

 cancellate appearance. 



This shell has a general resemblance in form to Orthis umhonata; but 

 it is a much larger species, has the beak proportionally more slender, the 

 cardinal extremities rounded, and the mesial sinus of the ventral valve 

 ( a marked feature in O. umhonata ) rarely defined and never conspicuous. 

 Geological j)osition and locality. In the dark shales of the Hamilton 

 group : Shores of Seneca lake below Lodi. 



