FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 85 



Rhynchotreta cuneata, var. americana. HALL. 



[Plate XXXII., figures 58, 59, 62 and 63. 



Rhynchotreta cuneata, var. americana, Hall. 28th Rep. N. Y. State Museum, Nat. Hist. Mus., p. 167 1879. 

 Rhynchotreta cuneata, var. americana, Hall, llth Geol. Rep. of Indiana, p. 310, pi. 25, figs. 29-38 1881 



Shell triangular, cuneiform, longer than wide, greatest width near the froni 

 and tapering posteriorly into an angular beak. Valves moderately convex, 

 the dorsal sometimes gibbous, ventral beak elongated, foramen sub- circular, 

 formed by the extremity of the beak, and a portion of the area below, which 

 is separated from the hinge- line by a deltidium in two pieces ; sides of the 

 beak compressed, flat or concave. Sinus wide, deep or shallow, according to 

 development of shell, commencing at one-third the length of shell from beak, 

 and becoming very conspicuous in front. 



Dorsal valve the more convex, the mesial fold beginning as a depression just 

 below the beak, and becoming very prominent on lower half of shell. Surface 

 marked by nine or ten strong angular plications on each valve, of which three 

 are depressed in the sinus, and four are elevated on the mesial fold, the two 

 central ones being much the more prominent; the plications are crossed by 

 numerous regular thread-like striae. The entire surface is minutely papillose. 

 (Hall.) 



Formation and Locality. Found in the Niagara strata east of the city of Louisville, Ky. There 

 it is, however, extremely rare, and the few specimens found are not well preserved, and show, besides, that 

 the condition of our Niagara ocean did not favor their development, which appears to have been poor. 

 Numerous, originally well developed, and afterwards well preserved specimens of this species come from 

 Waldron, Indiana. In that locality the Niagara sea must have afforded conditions most favorable to the 

 development of its rich molluscan fauna. If we compare this species with its European congener, we find 

 our shell more robust and larger, and its surface is marked by fewer and stronger plications ; while the 

 transverse striae are a little finer and less distant, and the sinus is much deeper and more abrupt. The 

 figures on plate 32 are copied from Hall's 28th Report. My own specimens from our locality were not 

 sufficiently preserved to be used for figures. 



Genus Ambocoelia. Haii. 



Shell bivalve, inequivalve, equilateral, plano-convex or concavo-convex ; 

 valves articulated by teeth and sockets ; cardinal line equalling the greatest 

 width of the shell. Area common to both valves ; foramen triangular, extend- 

 ing also to the area of the dorsal valve. Dorsal valve flat, depressed convex or 

 concave ; cardinal process bifurcate. The foveal plates are straight, longitudi- 

 nal, pointed at their inner extremities, and from their center on the outside 

 extends a callosity curving around the dental sockets, which open towards the 

 cardinal line. Muscular impressions four, and distinctly marked in the middle 

 of the valve or below. Ventral valve arcuate, with or without a distinct sinus ; 



