295 



most gibbous near the umbo ; beak small, closely incurved, filling the 

 foramen of the opposite valve. Both valves marked by four broad, 

 rounded plications, which become obsolete near the middle of tht 

 shell ; those of one valve opposing those of the other, causing an equal 

 number of projections on the front margin. The entire surface marked 

 by several strong concentric lines of growth. 



This species may easily be recognized by the peculiarity of the 

 opposite plications, which is a rather remarkable feature. 



Genus Pentamerus, Sowerby. 

 Pentamerus lenticularis (n. sp.) 



Shell small, broadly ovate, or subcircular; length and breadtli 

 nearly equal, profile lentiform. Valves subequal, depressed convex. 

 Beaks small, pointed, slightly incurved, sides and front regularly 

 rounded. Ventral valve a little the most convex ; the beak pointed . 

 and projecting beyond that of the dorsal. Spoon-shaped process of 

 the interior of the ventral valve proportionally large, in some speci- 

 mens nearly one-third the width of the shell, and extending about one- 

 third the length of the valve ; longitudinal septum reaching to neai' 

 the centre of the shell. Interior of dorsal valve with a single longi- 

 tudinal septum, with horizontal plates curving toward the cavity of the 

 opposite valve. Strong radiating muscular or vascular markings ap- 

 pear on internal casts of both valves. 



Surface apparently smooth, without mesial fold or sinus. 



This species has the general form of P. oUongus as it occurs in the 

 Clinton group of New York, but is quite small, seldom measuring more 

 than six-tenths of an inch in length. The internal characters of th(,- 

 ventral valve are precisely those of Pentamerus, but those of the 

 dorsal vary somewhat from those of most species of the genus, in 

 having but one longitudinal septum, with the crural appendages 

 spreading horizontally from it. 



Although species of this genus have been observed in the Old World, 

 in rocks as high even as the carboniferous formation, yet we believe 

 this is the first instance of their having been noticed in this country in 

 strata of a higher geological position than the Hamilton group of New 

 York. 



conchifera. 



Genus Aviculo-pecten, McCoy. 



AVICULO-PECTEN LIMAFORMIS (n. Sp.^ 



Shell below a medium size, subovate in outline, higher than wide, 

 greatest width below the middle. Valves very depressed, convex, the 

 left a little the most ventricose ; hinge fine short, less than half as wide 

 as the body of the shell. Auricular extensions small, flattened, and 



