40 PALAEONTOLOGY OF KENTUCKY. 



equal ; the ventral one depressed from center to base, while the other one is 

 regularly convex ; beak a little elevated above the other and slightly incurved ; 

 hinge-line much shorter than width of shell ; cardinal extremities rounded ; 

 hinge-area small. Dorsal valve slightly convex ; beak and hinge area small. 



Surface covered by fine radiating striae, which increase towards the front 

 by interpolation and bifurcation. The interpolated striae start at anterior part 

 of umbo, the bifurcation commences at the middle of the valves ; sometimes 

 the bifurcation is repeated or double ; concentric lines of growth divide the 

 surface of each valve into different concentric zones, which are more promi- 

 nent in younger than older individuals. In young shells the shape is more 

 ovate, in old ones more circular, besides the smaller specimens are more gib- 

 bous than the large ones. 



This species is as variable in size as Orthis elegantula ; it ranges from the 

 size of a pea to that of a silver half-dime and more. 



Formation and Locality. Niagara group east of the city of Louisville, Ky., where very perfect, 

 but also rery small specimens are pretty abundantly found. The largest shells of this species are found 

 at Waldron, Indiana. The individual represented on plate XXXII. is of the average size found near 

 Louisville. 



Orthis livia. BILLINGS. 



Plate XVI., figures 23 and 24; Plate XVII., figures 33, 34 and 35. 

 Orthis livia, Billings. Can. Jour, of Ind. Sci. and Art, No. 27 1860, 

 Orthis livia, Billings. Can. Jour., Vol. 5, p. 267 1860. 

 Orthis livia, Hall. Pal. N. Y., Vol. IV., page 381867. 

 Orthis livia, Billings. Pal. Fossils, Vol. II., pt. I., p. 321874. 



This species w;is established and first described by Mr. Billings, of the 

 Canadian Geological Survey. I will, therefore, only copy his description; his 

 figures correctly agree with mine. 



"Shell sub-orbicular or sub-quadrate; length about eight-ninths of the 

 width ; greatest width usually a little in front of the middle ; length of 

 hinge-line one-half to two-thirds the width of shell; cardinal extremities 

 rounded ; sides in most specimens somewhat straight, often sufficiently 

 curved to give a circular aspect to shell ; front angles obtusely rounded ; 

 front margin in general broadly convex, sometimes, in a small central por- 

 tion, nearly straight. Dorsal valve of medium convexity, most elevated about 

 the middle; the outline forming an uniform arch from depressed beak to 

 front margin ; slope from umbo to cardinal angles gently concave ; sometimes 

 a barely perceptible mesial depression, commencing in a point at the beak, 

 and becoming obsolete at one-half or two-thirds the length ; area small, lying 

 in plane of the lateral margins; beak minute, forming a small triangular 

 projection, rising scarcely one-fourth of a line above edge of area. Ventral 



