36 PALAEONTOLOGY OF KENTUCKY. 



Mesial fold prominent and well-defined from beak to front, somewhat flat- 

 tened on top, bearing five plications in its lower half. In younger specimens 

 the fold has only three plications, of which in older individuals the lateral ones 

 dichostomize at about the middle of shell, and thus form the five costae of 

 the lower part. The umbo is more, and the beak less prominent than that of 

 the other valve; hinge-area also somewhat smaller than the ventral. Size of 

 shell variable, the largest specimen in my collection, figured on plate XXIX., 

 sub-figures 18, 19 and 20, has a width of seven-eighths of an inch, a length of 

 five-eighths of an inch, by a depth of three-eighths of an inch. 



Surface marked by from five to seven strong angular plications, counted at 

 the margins, some of which are formed by bifurcation. No other surface-mark- 

 ings are visible. 



Formation and Locality. Pound in the lower strata of the Niagara formation in the quarries east of 

 Louisville, Ky. This species is very common and of larger forms in the Lower Silurian, but to find its 

 representatives in the Niagara rocks is somewhat of a surprise. They are rather belated stragglers. 



Orthis borealis. BILLINGS. 



Plate XXXIV., figures 14 to 20. 



Orthis borealis. Billings. Can. Nat., Vol. IV., p. 4361859. 

 Orthis frankfortensis, James. Cat. Low. Sill. Foss., Gin. gr., p. 10 1871. 

 Orthis borealis, Meek. Pal. of Ohio, Vol. I, p. 101., pi. 81873 



Shell of rather less than medium size, transversely oval-sub-quadrate, or 

 truncato-sub-oval, the length and breadth varying with relation to each other 

 from 9 to 11, to 11 to 12 ; both valves convex ; hinge-line generally less than 

 the greatest width of the shell, and meeting the lateral margins under a more 

 or less obtuse angle ; lateral margins, from cardinal extremities to base, form- 

 ing a compound, reversed curve of regular form, concave at the cardinal angles 

 and thence convex to base, which is broadly rounded, but sometimes showing 

 also a faint sinuosity. Ventral valve most convex near umbo, sloping from 

 there somewhat abruptly, but in a gentle curve to posteriolateral margins, while 

 the anterior central region is depressed so as to form a broad, very shallow 

 mesial sinus which extends backwards to middle of shell ; beak more prominent 

 than that of the other valve, sharply pointed, inclined backward and slightly 

 arched ; cardinal area broad-triangular, well defined by angular margins, and 

 concave ; foramen rather narrow, its height exceeding its base. Dorsal valve 

 sometimes more convex than the other; its greatest convexity a little above 

 the middle of the valve, from where it slopes in gentle curves to lateral and 

 antwior margins ; the central-anterior portion of the valve is slightly elevated, 

 forming a broad, low, and undefined mesial fold, whose starting point is at, or 

 somewhat in front of middle of valve ; beak of medium size, arched, but not 



