NATURAL SCIENCESOP PHILADELPHIA. 
409 
Ventral valve unknown. Dorsal valve with a blunt depressed beak, equalling 
the hinge, a moderately elevated umbo from which the surface slopes with 
gentle convexity to the lateral and anterior margins, and abruptly, with 
slight excavation, towards the superior portion of the rounded hinge-margins. 
Surface marked by about 32 fine rounded plications, which reach from the 
margin half way to the beak. Mesial fold wanting. Mesial septum extending 
one-fifth the length of the shell. 
Length of dorsal valve *25 (100) ; breadth *25 (100) ; convexity -08 (34). 
Locality.— Grindstone quarries, P. aux Barques, with R. camerifera. 
This species is a close analogue of R. radialis, Phillips, sp. (Greol. Yorks. 
223, pi. xii. 40, 41) from the carboniferous limestone of Bollard. 
Rhynchonella Huronensis, n. sp.—Shell of medium size, tumid, transversely 
oval, or nearly circular, with rounded lateral, and cuneate rostral margins. 
Ventral valve with a straight beak, flattened in the central region, and rather 
abruptly inflected around the margin, toward the plane of the valve; mesial 
sinus beginning with the last third of the shell-length, and consisting of a 
sudden depression in the antero-marginal slope. Dental lamellse well devel¬ 
oped, very slightly divergent. Dorsal valve with an inconspicuous beak and 
a mesial fold abruptly elevated and confined to the anterior third of the valve. 
Median septum reaching two-fifths the length of the valve. Oeclusor muscu¬ 
lar impressions, semi-elliptic, lying close to the median septum. Shell-struc¬ 
ture fibrous. Surface marked with 23 small rounded ribs, of which five occupy 
the mesial sinus. 
Length of the ventral valve *48 (100); breadth *58 (121); convexity *10 
( 21 ). 
Locality. —Light-house, Pt. aux Barques, in a hard pyritous sandstone in¬ 
tercalated in the argillaceous slates of the Huron group. 
Var. precipua differs from the typical forms in being more flattened on the 
ventral side, with mesial sinus consisting of an abrupt deflection of nearly the 
whole anterior margin of the valve, forming a right angle with the plane of the 
valve; surface with 18 rounded radiating ribs, of which 6 fall in the sinus ; 
dental plates diverging at an angle of 40°. 
Orthis, Dalman. 
Orthis Vanuxemi, Hall (10th Ann. Rep. N. Y. Reg., p. 135).-Shell nearly cir¬ 
cular, sub-tumid ; hinge-line very short. Dorsal valve a segment of a sphere ; 
beak not surpassing the hinge, slightly incurved; a thick median plate or 
ridge reaching nearly to the centre of the valve, bisecting the right angle 
formed by the well developed socket ridges. Ventral valve flat, or slightly 
concave anteriorly, with a projecting beak; median ridge feeble, extending 
scarcely to the mid-valve; a barely perceptible trace of the semi-circular 
divaricator impressions sweeping from the beak to the anterior extremity of 
the median ridge, in the middle of which space are the two small semi-elliptic 
oeclusor scars ; dental plates short and thick ; teeth well developed, lying in 
the hinge-line. One of the casts differs in having one of the oeclusor scars 
half heart-shaped and the dental plates more slender. Surface not fully 
known; marked by numerous radiating strise which increase by implantation 
and bifurcation, and produce a crenulated anterior margin. Shell structure 
finely punctate. 
Length *81 (100) ; breadth *81 (100) ; thickness *25 (31). 
Locality. —Light-house, Pt. aux Barques, with Rhynchonella Huronensis , Syc. 
This shell is a little more convex in the dorsal and flatter in the ventral than 
the figures given by Prof. Hall, but none of its characters differ materially 
from his description. Compared with O. Michelini, Lev., as described by de 
Koninck, it is a little more convex dorsally, and presents circular instead of 
digitate [from the vascular system?] divaricator impressions upon the ventral 
1862 .] 
