FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 73 



valve, but the growth of the deltidium usually renders it complete in adult 

 shell ; in the cretaceous species it is tubular. In some species the beak is so 

 closely incurved upon the other as to allow no space for a pedicle. 

 The Rhynchonella loxia is made the type of the genus Rhynchonella. 



Rhynchonella acinus. HALL. 



Plate XXVI., figures 6, 13 and 14, and Plate XXXII , figures 13 to 16. 

 Rhynchonella acinus, Hall. Trans. Alb. Inst., Vol. IV., p. 2151863. 



Rhynchonella acinus, Hall. 28th Rep. N. Y. St. Mus. Nat. Hist., Mus. edit., p. 163, pi. 26, figs. 7-111879. 

 Rhynchonella acinus, Hall, llth Geol. Rep. of Ind., p. 3061881. 



Shell very small, longitudinally ovate, sub-attenuate towards the beak and 

 truncate in front, valves sub-equally convex. Ventral valve sub-arcuate, flat- 

 tened in the middle, below which it is sinuate ; beak incurved. Dorsal valve 

 somewhat flattened in the middle, and sometimes a little depressed in upper 

 part of median line, two of the plications becoming elevated towards the front, 

 corresponding to an abruptly depressed sinus in ventral valve, in the bottom 

 of which is a single plication ; there are three and rarely four plications on 

 each side of mesial fold of dorsal valve, and four on each side of the sinus of 

 the ventral valve. Concentric lines of growth usually only faintly marked. 

 Length to width usually about as four to three, and depth about equal to 

 width, giving a sub-quadrate transverse section ; average size of the shell as 

 represented in the figures on plate 26. 



This species differs from Rhyn. bidentata of Hisinger, in being larger, more 

 robust and .ventricose, and proportionally more elongate; the plications are 

 more rounded, and the whole aspect of the shell less angular. It approaches 

 in form the Rhyn. bialveata of the lower Helderberg formation, but it is 

 more robust, and the plications are more rounded. (Hall.) 



Formation and Locality. Found in the lower strata of the Niagara group in the quarries east of 

 the city of Louisville, Ky., but of rare occurrence. On some of the plates the two lower figures of this 

 species have no numbers. The side view should be number 13, and the front view number 14. This front 

 view shows in the mesial elevation three plications ; there should be only two. 



Rhynchonella bellaforma. K. SP. 



Shell of medium size ; sub -triangular or sub-pentagonal ; moderately convex, 

 rather somewhat compressed ; cardinal margins sloping down to the middle of 

 shell, and forming at the beaks an obtuse angle of about one hundred degrees ; 

 lateral margins short and strongly curved ; base only curved at the front 

 angles, the balance, about four-fifths of the whole front, almost straight, with 

 a slight emargination or inflection of its middle part. Width to length as 

 three to two ; greatest width below the middle. 



Ventral valve moderately convex at the umbonal region, and near the cardinal 

 and lateral margins ; in front of the umbo the central portion of the valve 



OEOL. 8UR. 10 



