336 
PALAEONTOLOGY OF NEW-YOLK. 
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 striae, which are undulated in crossing 
the plications, mark the surface of the shell; but this feature is rarely 
preserved. ( 
Some specimens from the Corniferous 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 
iS. sappho of the Hamilton group. 
Geological formation 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. 
Ithyiichoiiella (Stenocisma ) hillings! (n. s.). 
PLATE LIY. 
Rhynchonella thalia : Billings, Canadian Journal, p. 272. 1860. 
Not R. thalia : D’Oebigny. 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. 
