FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 91 



points of greatest convexity, equals the width ; hinge-line short and sloping 

 from the beaks with a forward deflection to the cardinal extremities, which are 

 rounded ; lateral margins slightly curved or almost straight ; basal margin 

 strongly curved and sometimes even forming a pointed basal extremity. Sur- 

 face covered with plications. 



Ventral valve less convex than the other ; greatest convexity at the umbo, 

 from which it slopes in a gentle curve to the front, and to the basal half of 

 the lateral margins, but more rapidly, almost abruptly, to the cardi no-lateral 

 margins. The umbo is prominent, and the beak somewhat elevated above, 

 but incurving closely upon the umbo of opposite valve. No hinge-area nor a 

 perforation in the beak are visible. The dorsal valve is more gibbous, it slopes 

 in a regular, light curve to the base, but in a considerably stronger curve to the 

 lateral and cardinal lines ; its umbo is prominent, and its beak is closely in- 

 curved into the opposite valve. It is only near, almost at the base, where the 

 dorsal valve shows a faint elevation, indicating a mesial fold, and the ventral 

 valve a slight impression in place of a mesial sinus. The dimensions of an 

 average-sized specimen are : length, nine-tenths of an inch ; width and depth, 

 six-tenths of an inch. 



Formation and Locality. In the Corniferous limestone at the Falls of the Ohio, on the Indiana 

 shore of the river. 



Atrypa reticularis. LINNAEUS. 



Plate XIV., figures 12 to 23, and Plate XV., figure 1. 

 Atrypa reticularis, Linnaeus. Syst. Nat. ed. XII., p. 1132 1767. 

 Atrypa reticularis, Hall. Pal. N. Y., Vol. IV., p. 3161867. 

 See list of synonyms in last cited report. 



Shell large, sub-circular or ovoid, convex-concave, but mostly plano-convex ; 

 length and width almost equal, sometimes width a little greater than length; 

 surface plicated; hinge-line less than width of shell ; cardinal extremities 

 rounded, lateral margins gently and regularly curved ; basal margin forming a 

 somewhat stronger curve, which is sometimes interrupted by a small extension 

 of the mesial sinus, but oftener by a slight inflection, caused also by sinus. 

 Dorsal valve very ventricose, sometimes gibbous, having greatest convexity 

 either at the umbo or in front of it, from where it slopes abruptly to the cardi- 

 nal and lateral margins, but more gently to the front. In some shells there is 

 a slight elevation in the center line of the valve from beak to base, indicating 

 a kind of mesial fold, but in most specimens this feature does not exist ; the 

 umbo is prominent and regularly rounded ; beak strongly incurved into the 

 fissure of the other valve. Ventral valve depressed convex in the upper part 

 near the umbo, but becoming concave on the sides and in front of it. A broad 



