FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 43 



apex projecting backwards over the area. The elevation about equal to half 

 the length of the dorsal valve ; area twice as wide as high, fissure very nar- 

 row and slender, and reaching to the apex. Surface marked by strong angular 

 striae, which are increased by interstitial additions, to the number of twenty- 

 eight or thirty, on the margin of the shell ; striae crossed by distinct lines of 

 growth. 



Length, five lines on the dorsal valve ; width, seven lines. This species is 

 of the type of Orthis tricenaria, of the Trenton group, but its beak is much 

 more elevated, its area higher, and its striae more angular, and these are 

 increased by interstitial additions, while in O. tricenaria and its' congener O. 

 pectinella the striae are simple. 



Formation and Locality. Found in the upper strata of the Niagara formation in the quarries east 

 of the city of Louisville, Ky., where well preserved specimens are, however, rather rare. 



Orthis propinqua. HALL. 



Plate XVI., figures 1, to 3, and 7 to 11. 



Orthis propinqua, Hall. Tenth Rep. on St. Cab., p. 1101857 

 Orthis propinqua, Hall. Pal. N. Y., Vol. IV., p. 431857 



Shell of more than medium size ; transversely elliptical or sub-quadrate ; 

 varying from moderately convex to gibbous ; hinge-line about one-half the 

 width of shell ; cardinal extremities broadly rounded ; lateral margins regu- 

 larly curved beyond the front angles ; basal margin broadly curved and trun- 

 cated or even somewhat inflected in its central part. 



Ventral valve less gibbous than the dorsal ; very prominent at the umbo, 

 sloping very abruptly towards the cardinal angles, but more gently towards 

 lateral and baso-lateral margins ; flattened in center ; the lower half marked 

 by a gradually increasing, broad and undefined sinus, which gives a strongly 

 upward curved outline to basal or front margin ; beak acutely pointed and 

 slightly incurved ; cardinal area elevated and concave, and divided by a tri- 

 angular foramen, which is open, and twice as high as wide in its base. The 

 dorsal valve is larger and more gibbous than the opposite one ; its greatest 

 convexity fs a little above the middle of the length, curving abruptly to the 

 cardinal and cardino-lateral margins, and somewhat more gently to the front 

 and baso-lateral margins. The cardinal extremities are sometimes slightly 

 deflected, and the surface at the cardinal angles a little concave. Cardinal 

 area is inclined to that of the ventral valve, and about one-half or two-thirds 

 as wide. Surface marked by fine, unequal, radiating striae, which increase in 

 number by intercalation, and are crossed by fine concentric striae, and, at un- 

 equal intervals, by stronger imbricating, lamellose lines of growth. 



This species resembles closely O. multistriata of the Lower Helderberg 



