FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 81 



2. The elevation and depression of the central plications are scarcely notice- 

 able in this shell. 



3. The beak of the ventral valve is more pointed, and elevated above that of 

 the other valve. One of my specimens of this species shows faint traces of 

 very fine radiating lines, also of fine concentric striae or lines of growth ; both 

 these lines, the radiating and the concentric, may also be found in more per- 

 fect specimens of Rhyn. saffordi, though none among the many shells of that 

 species in my collection show them. 



Formation and Locality. Associated with Rhyn. saffordi in the Niagara rocks in the quarries east 

 of the city of Louisville, Ky., but of rare occurrence. 







Rhynchonella Strickland!. SOWKRBY. 



Plate XXV1L, figures 9, 10 and 11, and Plate XXIX., figures 3 to 6. 

 Terebratula Stricklamli,, Sowerby. Sil. Syst., pi. 13, fig. 191839. 

 Rhyn. Tennesseensis (Koemer), Hall. Trans. Alb. Inst., Vol. IV. 

 Rhyn. Siricklandi, Hall. 28th. Kep. N. Y. State Museum 1876. 

 Rhyn. Stricklandi, Hall, llth Geol. Kep. of Ind., p. 308, pi. 261881. 



Shell one of the large-sized Rhynchonellidae, sub-ovate or sub-trigonal; width 

 sometimes equal, but mostly exceeding the length ; sides and front rounded ; 

 shell almost concavo-convex. Surface plicate. 



Ventral valve regularly curved from beak to front j the mesial sinus, which 

 commences below the umbo, occupies, from that point to the front, the largest 

 portion of the valve ; it is shallow at the beginning, but deepens approaching 

 the front. On each side of the sinus are five plications smaller than those 

 occupying the mesial depression ; in the most lateral plication the margin of 

 the valve makes a very abrupt bend, equalling . or exceeding ninety degrees 

 towards the dorsal valve, forming here a smooth, vertical border ; beak small 

 and incurved over the umbo of the other valve. The sinus contains usually 

 nine strong, simple and rounded plications ; the whole valve has from eighteen 

 to twenty of these ribs, which increase in size and distance from beak to front ; 

 most all of them reach to the front, only a few appear to be somewhat shorter. 

 Dorsal valve is very gibbous or ventricose, slightly flattened at the umbo ; 

 mesial fold not defined on the umbo ; from there it becomes gradually more 

 elevated as it approaches the front, where it is more or less prominent. The 

 fold has generally four plications on top and three on each of its slopes ; 

 outside of the mesial elevation, each side of the valve contains four plications, 

 thus covering the whole valve with about eighteen ribs ; those on the fold are 

 somewhat stronger than the lateral. In the last lateral plication the surface of 

 the valve bends abruptly towards the ventral valve, forming here, like the latter, 

 a smooth border of considerable size. All the ribs are roundect and simple, 



OEOL. SUR. 11 



