NATURAL SCIENCESOF 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 lamellae 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. Occlusor 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. 



Obthis, 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 

 occlusor scars ; dental plates short and thick ; teeth well developed, lying in 

 the hinge-line. One of the casts differs in having one of the occlusor scars 

 half heart-shaped and the dental plates more slender. Surface not fully 

 known ; marked by numerous radiating strife which increase by implantation 

 and bifurcation, and produce a crenulated anterior margin. Shell structure 

 finelv punctate. 



Length -81 (100) ; breadth -81 (100) ; thickness -25 (31). 



Locality. Light-house, Pt. aux Barques, with Rhynchonella Hwonensis, frc. 



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. Michel ini, 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 



18.62.] 



