RHYNCHONELLID^ OF THE HAMILTON GROUP. 339 



and gently defined below the middle of the length of the shell. The 

 umbo is prominent, the beak small and neatly incurved over the umbo 

 of the opposite valve. 

 Dorsal valve much more convex than the ventra\, the greatest gibbosity 

 a little above the middle, gently curving to the sides and baso-lateral 

 margins ; the centre elevated in a broad mesial fold. 

 Surface marked by numerous rounded or subangular plications, of which 

 from three to six or seven are depressed in the sinus and a correspon- 

 ding number elevated on the fold. "The plications of the fold and sinus 

 are always bifurcating, those of the sides simple or obscurely bifur- 

 cating, and all usually becoming obsolete at one-third to one-fourth the 

 length of the shell from the apex ; concentrically marked by raised 

 thread-like striae, which sometimes become squamose imbricating 

 folds or lines of growth. 



The cast of the ventral valve shows a narrow rostral cavity and slen- 

 der dental plates, with a lanceolate or lance-ovate muscular area. The 

 ovarian spaces are papillose-striate, and outside of these are the rami- 

 fying vascular imprints. The cast of the dorsal valve shows a median 

 septum reaching more than half the length of the shell, divided above, 

 leaving a triangular pit. The muscular imprint is narrow, elongate, and 

 marks the surface on each side of the septum more than half-way to the 

 base. 



This species presents considerable variety in its surface characters, varying 

 from specimens with few perceptible plications on the sides of the shell, to those 

 with six or seven on each side, and from three to seven in the mesial sinus. In 

 some of the specimens in the Hamilton shales, the plications are numerous, suban- 

 gular, reaching to the apex of the shell, and showing distinct bifurcations in 

 those of the lateral as well as the mesial portion of the valve. 



In order to make a comparison with the Canadian species, I have procured 

 specimens from Bosanquet and Widder in Canada West ; and I find the same 

 variations among them as in those of New-York. In seventeen specimens examined, 

 there were two corresponding essentially with the figure of Mr. Billings (loc. 

 cit.), except that the plications are obsolete towards the beak : one of these 

 specimens is about an inch in length and breadth, and the other a little less. In 

 other specimens of nearly the same size, there are four or five plications on the 



