550 Contributions to Invertebrate Palaeontology. 



tration ; still, however, sufficiently distinct to leave no reasonable 

 doubt of their identity. They are smaller in size than those from 

 the Upper Helderberg limestone of Canada, but otherwise not 

 different so far as can be discovered from the imperfect material on 

 hand. 



Genus LEIORHYIVCHIJS Hall. 



Leiorliyiicliiis limitaris. 



Plate XI, fig. 11. 



Orthis limitaris Vanuxem, Geol. Rept. 3d Dist. N. Y., 1842, p. 14(5, fig. 3. 

 Atri/pa limitaris Hall, Geol. Rept. 4th Dist. N. Y., 1843, p. 182, fig. 11. 

 Leiorhynchus liiJiitai-is Hall, 13th Rept. State Cab., p. 85, 1860. 

 Leiorhynchus limitaris Hall, Pal. N. Y., vol. 4, p. 356, pi. 56, figs. 6-21. 



Shell small in size, seldom exceeding five-eighths of an inch in width and 

 usually not more than three-eighths ; form orbicular in outline and lenticular 

 in profile when not compressed. Valves subequal in depth and rotundity, the 

 ventral beak slightly extending beyond that of the dorsal and the middle 

 third or more of the width of the valve depressed, forming a broad but shallow 

 sinus which extends to within a short distance of the beak. Dorsal valve 

 elevated in the middle to form the fold which corresponds to the sinus of the 

 ventral, but which does not continue much beyond the middle of the valve. 

 Surface of the shell marked by from ten to twelve or more low, angular plica- 

 tions, four or five of which are seen in the sinus of the ventral, and a corre- 

 sponding number elevated on the fold of the dorsal valve, and from three to 

 four or even five mark each side of the shell beyond the limit of the fold and 

 sinus. The specimens are usually marked also by several strong concentric 

 lines of growth which form strong varices on the larger specimens, and the 

 plications are not unfrequently divided by slighter depressions along their 

 surfaces, which gives them the appearance of being bifurcated, and the plica- 

 tions themselves are very unequal in strength and seldom extend entirely to 

 the apex of the valves. 



This shell is a very well-marked species and cannot well be mis- 

 taken for any other of the several species, which, so far as is yet 

 known, are limited to certain horizons; this one characterizing the 

 horizon of the Marcellus shale in New York, wherever the species 

 has been found. Its occurrence in Ohio has not heretofore been 

 known or suspected, and its presence in numbers, flattened and 

 compressed in a dark brown, somewhat fossil shale, presenting so 

 exactly the characters and appearance that it does in the shales of 

 New York, and also associated with other characteristic forms of 

 the Marcellus shale, is a somewhat significant fact, and one of con- 

 sideraljle importance in its stratigraphical relations. 



