62 SYNOPSIS OF THE CHARACTERS OF THE 



These beautiful sheUs are remarkable for having the anterior half of their valves marked with strong, trans- 

 verse ridges, and the posterior half perfectly smooth; besides tliis circumstance they are distinguished from Cy- 

 pricardia, with which I formerly arranged tliem, by the thiniiess of their shells. They seem to have much the 

 habit of Saxicava, Lam., or more properly, perhaps, oi Leptodomus, M'Coy. I have dedicated this genus to 

 Professor Sedgwick. 



Sedgwickia attenuata. M'Coy. (PI. XI. fig. 39)- 

 Sp. C/i.— Transversely ovate, convex ; beaks large, compressed; anterior side very short, rounded; poste- 

 rior side narrow, lengthened, subtruncate, rounded ; hinge-line as long as the shell is wide ; anterior side and 

 middle of the shell marked with about fifteen strong, transverse ridges, remainder of the surface smooth. 



This shell resembles the S. buUata, M'Coy, but is less convex, larger, and has a longer hinge-line, in con- 

 sequence of which the truncation of the posterior end is not at all oblique ; it also resembles the S. gigas, 

 M'Coy, but is much smaller, has much fewer transverse ribs, and is more elongated transversely. Length seven 

 lines, width eleven lines. 



Sedgwickia bullata. M'Coy. (PI. VIII. fig. 19). 



Sp. Ch Transversely oblong, very gibbous, length about two-thirds the width; anterior side short, 



rounded ; hinge-line nearly as long as the shell is wide ; posterior end slightly obhque, subtnmcate ; beaks very 

 large, tumid ; an obtusely rounded ridge, from the beak to the posterior angle ; anterior side with strong, 

 rounded, distant ridges ; middle of the shell and posterior side smooth. 



The substance of the shell of this handsome species is remarkably thin. Length four and a half lines, 

 width six and a half lines. 



Sedgwickia corrugata. M'Coy. (PL VIIL fig. 18). 



Sp. C/<.— Transversely elongate, twice as wide as long, gibbous; a strong, diagonal, rounded ridge, from 

 the beak to the posterior angle ; surface smooth, with the exception of four or five strong, transverse wrinkles 

 on the body of the shell. Length one and one-third lines, width three lines. 



Sedgwickia gigantea. M'Coy. (PL XI. fig. 40). 



Sp. Ch. — Rhomboidal, ovate, convex, depressed; beaks very large, subcentral, nearest to the anterior end; 

 anterior side rounded, abdominal margin very convex ; posterior side obtusely rounded, subtruncate ; anterior 

 end marked with about forty sharp, transverse ridges, remainder of the surface smooth. 



This fine shell is very much the largest of the group, and is distinguished besides by the great number of 

 the anterior sulci. Length one inch nine lines, width two inches nine lines. 



Sedgwickia globosa. M'Coy. (PL XI. fig. 38). 



Sp. Ch. — Orbicular, globose, beaks nearly central; surface smooth, except on the anterior side, which is 

 marked with short, delicate, transverse ridges. 



This small species is remarkable for its orbicular form and great convexity. Length three lines, width 

 three and a half lines. 



Sedgwickia MINIMA. M'Coy. (PL VIII. fig. 17)- 



Sp. Ch. — Transversely elongated, subtrigonal, very gibbous; beaks large, prominent; anterior side small, 

 obtusely pointed ; posterior end very broad, subtruncate, rounded ; the greatest gibbosity is obliquely from the 

 beak to the anal angle, but there is no distinct ridge ; posterior end smooth, anterior end marked with short, 

 strong, transverse strise. 



