FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 207 



The posterior margin slopes down from the hinge-line, either at right-angles, 

 or deflecting backwards, and forms a gentle curve ; the basal line is very short, 

 and strongly curved, forming almost a linguiforra extension of the shell ; its 

 most prominent point is either entirely posterior, or below the center of the 

 posterior portion of the hinge-line. 



Both valves have their maximum convexity a little above the middle of the 

 shell, from where they slope in a regular curve, which is more or less strong, 

 according to the greater or smaller gibbosity of the shell, towards the sides 

 and front ; the slope on the posterior side of the umbonal region appears to 

 be somewhat steeper than the anterior, causing the flattening of the shell along 

 the upper half of the posterior margin, and especially at the posterior cardinal 

 angle. The umbonal region extends from the beak to the base, and crosses the 

 valves in a somewhat diagonal direction, terminating at the basal extremity. 

 The umbones are markedly deflected towards the anterior end ; they are only 

 moderately elevated above the hinge-line; the beaks small and closely incurved; 

 the hinge is nearer to the anterior end, at about two-fifths of the whole length 

 from the front. The size of the species is shown by the two specimens illus- 

 trated on plate 34, of which one is only moderately ventricose, while the other 

 is very gibbous. 



The surface is marked by closely set, strong, concentric lines of growth, 

 which are more conspicuous near the margins than on the umbo. 



Formation and Locality. Found by me in the Hudson River shales of Oldham county, Ky. 

 Outside of the two specimens belonging to my own collection, I do not know of any others. To contribute 

 my mite, small as it may be, to the many well deserved honors of America's greatest geologist and palaeon- 

 tologist, the venerable Prof. James Hall, of Albany, New York, I have selected his name for this beautiful 

 little shell. 



Genus Grammysia. Devemeun. 



Qrammysia, De Verneuil. Bull. Soc. Geol. France 1847. 



Etymology : gramme, a line of writing; mys, a mussel shell; in allusion to the transverse furrows which 

 cross the valves from the umbones to the middle of the ventral margin. 



Shells equi valve, inequilateral, not gaping, furnished with two muscular im- 

 pressions of very unequal size; pallial line rounded posteriorly, and united with 

 the large muscular impression in such a manner as to leave about two-thirds of 

 it outside of the line ; ligament external, much prolonged in the depression of 

 the escutcheon. 



Surface traversed by an oblique fold or rib extending from the beak to mid- 

 dle of the inferior border, and by numerous rounded concentric folds. This is 

 De Verneuil' s description, which is improved upon by Prof. Hall's, which is as 

 follows : Shell equivalve, inequilateral, varying from sub quadrate to trans- 



