MERISTELLA OP THE UPPER HELDERBERG GROUP. 3»3 



It differs from M. harrisi, in being less gibbous and proportionally broader. It 

 is more elongate than M. haskinsi, less rounded in the outline of the lower part, 

 and does not present the conspicuous rounded foramen of that species. It has not 

 proved a common species in the limestone, only a few specimens being known. 



Geological formation and localities. This species occurs in the Corniferous lime. 

 stone at Williamsville and Clarence-hollow, Erie county, N. Y. 



Meristella doris. 



PLATE L. 



. Meristella doris : IlAti, Thirteenth Report on the State Cabinet, p. 84. 1860. 

 Charionella doris : Billings, Geology of Canada, p. 374. 1863. 



Shell subovate, elongate, gibbous above and more or less compressed 

 below the middle, with the anterior and antero-lateral margins 

 attenuate and sharp ; sinuate on the ventral side below the middle, 

 and the front produced in a broad extension. 



Ventral valve a little more or less convex than the dorsal, gibbous above 

 the middle : a mesial depression, often beginning about the middle of 

 the length, becomes broader and deeper towards the front, sometimes 

 affecting nearly the entire width of the shell. This sinus is sometimes 

 barely perceptible, and is sometimes a narrow, almost linear depres- 

 sion in the upper part of the shell. The umbo is prominent and the 

 beak strongly incurved : an angular ridge extends from this point 

 obliquely on each side to below the extremities of the hinge-line, on 

 which the shell is bent abruptly (and on the upper part acutely) 

 inwards. The central upper portion of the valve is prominent, and 

 between this and the oblique ridge the surface is depressed or flat- 

 tened. The apex is usually apparently closed, but from accident or 

 otherwise it often presents a circular foramen : the triangular space 

 beneath is closed by deltidial pieces, or sometimes open. 



Dorsal valve sometimes more convex than the opposite, usually gibbous 

 in the middle and above, and curving rather abruptly towards the 

 lateral margins, in the older shells it becomes a little flattened or even 

 concave near the edge. The central elevation continues to the frou 

 without becoming a distinct mesial fold. 



