FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 83 



occupied by about seven, while the sinus only contains about six. These striae 

 increase by intercalation, but not by bifurcation. Other markings of the 

 surface are not observed. 



Formation and Locality. Occurs very rarely in the Corniferous rock of the Devonian formation 

 in Kentucky and Indiana. 



Rhynchonella tethys. BILLINGS. 



Plate XIII., figures 25 to 33, and Plate XXXI., figures 22 to 25. 

 Rhynchonella tethys, Billings. Canadian Journal, p. 270 1860. 

 Rhynchonella tethys, Hall. Pal. N. Y., Vol. IV., p. 3351867. 



Shell of medium size among ths Rhynchonellidae, sub- trigonal ovate, usually 

 wider than long, but sometimes length and width equal. Ventral valve de- 

 pressed convex in the upper part, nearly flat at the sides; sinus, beginning 

 above the middle, moderately depressed in young shells, and in older ones 

 becoming deep in front and abruptly curving towards the dorsal valve. Dorsal 

 valve moderately convex in young shells, more convex and finely gibbous in 

 old shells ; the broad mesial elevation becoming distinct about the middle of 

 the length. Surface marked by from fifteen to eighteen angular plications, 

 of which four to six are upon the mesial fold, and three to five in the median 

 sinus, while about five or six cover the lateral slopes of each valve on each side 

 of the mesial fold or sinus ; plications outside of fold on dorsal valve curve 

 very abruptly outwards as well as downwards. Fine concentric striae, which 

 are undulated in crossing the plications, mark surface of shell, but this feature 

 is rarely preserved. 



Formation and. Locality. Found abundantly and in well preserved specimens in the Corniferous 

 rock at and around the Palls of the Ohio, however, all the shells found are in a silicified condition, by 

 which, generally, the finer surface-markings become obliterated. If we compare the shell figured on plate 

 31 with those on plate 13, it appears to us that it does not belong to the same species. The shell on plate 

 31 has its plications finer and rounded, and its mesial elevations and depressions less prominent; it has six 

 ribs on the fold and five in the sinus. The shells on plate 13 show four to five plications on the fold, and 

 three to four in the sinus; all the ribs are coarse and angular. But if we compare the different shells in a 

 large lot of from fifty to sixty individuals, as are now before me, it is impossible to draw a dividing line; 

 there are numerous intermediates connecting the two extreme forms, which must, therefore, be classed in 

 one and the same species, 



Rhynchonella increbescens. HALL. 



Plate XXXIV., figures 26-29. 

 Atrypa increbescens, Hall. Pal N. Y., Vol. I., pi. 33, p. 1461843. 



Average shell small, ovate-trigonal, full-grown specimens quite convex, with 

 slightly greater width than length. Cardinal slopes nearly straight in young 



