336 PALAEONTOLOGY OF NEW- YORK. 



Dorsal valve moderately convex in young shells, more convex and finally 

 gibbous in old shells : the broad mesial elevation becoming distinct 

 about the middle of the length. 



Surface marked by from fifteen to eighteen angular plications of which 

 five or six are upon the mesial fold and sinus, and about an equal 

 number, or sometimes six or seven, on each side - the plications on 

 the dorsal valve near their extremities curve very abruptly to the 

 ventral valve. Fine concentric stria), which are undulated in crossing 

 the plications, mark the surface of the shell ; but this feature is rarely 

 preserved. 



Some specimens from the Coruifcroiis limestone of Western New- York appear 

 to be of this species, but they are smaller than the figures of Mr. Billings. A 

 single large specimen from Port Colborne, Canada West, has five plications in 

 the sinus and six on the fold, with seven on each side. 



I have received from Dr. Knapp of Louisville several fine specimens from the 

 limestone of the Falls of the Ohio, which I have referred to this species : one of 

 these has a length of eleven-sixteenths of an inch, with a width of nearly twelve- 

 sixteenths and a depth of half an inch. In another, the length, width and depth 

 are as 8, 10 and 5 respectively. A young individual has three plications in the 

 sinus and five on each side ; while a smaller one has but two plications in the 

 sinus. I have been inclined to believe that this is only a different condition of 

 S. sappho of the Hamilton group. 



Geological formaiion and localities. This species occurs in the Corniferous lime- 

 stone, at Stafford, Genesee county, and Williamsville, Erie county, New- York ; . 

 at the Falls of the Ohio ; at Port Colborne and elsewhere in Canada West. 



Rhynclionella ( 8teiiocisiiia ) billingsi (n. s.). 



PLATE LIV. 



Rhynchonella tkalia : Billino8, Canadian Journal, p. 272. 18C0. 

 Not R. thalia . D'Obbiosy. 1847. 



Shell small, ovoid subtrigonal, truncated in the middle of the front and 

 rounded on each side ; width greater than the length. 



Ventral valve moderately convex ; mesial sinus beginning a little above 

 the middle of the length of the shell, and marked by " three simple, 

 acutely angular ribs " ; 'beak elevated and acute. 



