418 GEOLOGY OF OHIO. 



scarcely perceptible angularity just posterior to the middle of the length. Surface 

 prominently convex and a little the fullest anterior to the middle ; ocular tubercle 

 small, situated a little below and just behind the anterior extremity of the hinge- 

 line. Lower margin of the valves slightly inflected, and in some cases the posterior 

 margin appears to have been bordered by a slightly thickened rim. 



The individuals examined are either internal casts or impressions of 

 the exterior, owing to which fact the finer surface features of the crust 

 cannot be definitely ascertained; enough is seen however to show its 

 identity with those from the Tentaculite limestones of New York. The 

 species as described by Mr. F. B. Meek includes this and the following 

 one, which are very distinct species, the differences being very strongly 

 marked in the great prominence of the lower part of the valves of that 

 one, and its strongly sub-angular form as well as in its greater size. The 

 principal variation noticed among the individuals of this species, is in the 

 greater proportional length of some of them, producing a cylindrical 

 form. This feature is however seen occasionally among those from 

 Schoharie, N. Y., but does not appear to be worth}' of specific considera- 

 tion. 



Formation and Locality. — In the hydraulic limestone of the Lower 

 Helderberg group, at Bellevue, Sandusky Co., Ohio. 



Leperditia angulijera. 



Plate I, figs. 28-30. 

 Leperditia angulifera Whitf., Ann. N. Y., Acad. Sci., March, 1882, 196. 



Carapace of medium size, having a length, in adult individuals, of about three- 

 eighths of an inch, by a height of one-fourth of an inch in the broadest part. Gen- 

 eral form of the outline broadly sub-ovate and widest posteriorly ; hinge-line 

 straight, equal in length to two-thirds that of the entire valve; anterior end a little 

 the shortest, narrowly rounding into the broadly curved basal line; posterior end 

 broadly rounded. Surface of the carapace highly elevated and prominent, forming a 

 strong, somewhat angular, longitudinal node just within the basal margin, and near 

 the middle of the length. From this point, the surface slopes somewhat gradually 

 upward to the hinge-line, with a barely perceptible convexity, except on the anterior 

 end, where it is more strongly convex, and characterized by a rather prominent and 

 well-marked ocular tubercle. From the angular node near the lower margin, there 

 is, on well-preserved individuals, a perceptible angulation, extending along the sur- 

 face to the point of greatest length on the anterior end, and a similar one, but less 

 strongly marked, on the posterior side. There is no perceptible difference in form 

 between the right and left valves, each showing the features about equally devel- 

 oped. No appearance of striations radiating from the ocular tubercle can be 

 detected, either on the internal casts or in the matrices ; still the nature of the rock 

 in which they are imbedded is such that very obscure markings would scarcely be 

 preserved. 



This species differs from Leperditia alta Conrad, of the same forma- 

 tion, in its larger size, and in the larger and more distinct eye-tubercle, as 

 well as in its slightl} 7- different position; but most distinctly in the sub- 

 angular ridge-like node, and greater convexity of the lower border of 

 the valves. This projecting node being situated near the lower margin, 



