INVERTEBRATES. 259 



nent than towards the front. Left valve moderately convex ; 

 anterior ear small, compressed, separated from the swell of the 

 umbo by a rounded depression, nearly rectangular, or some- 

 times denned by a very faint sinuosity in the margin below ; 

 posterior ear larger than the other, compressed, but not very 

 distinct from the umbonal slope, extending out into an acute 

 point beyond the posterior margin below, from which it is 

 separated by a broad rounded sinus; beak small, pointed, 

 slightly oblique, incurved, and extending a little beyond the 

 hinge margin; surface ornamented by about twenty-six simple, 

 sharply elevated, linear, radiating costse, separated at the mid- 

 dle of the valves, by spaces from three to five times their own 

 breadth, and crossed by similar regularly disposed concentric 

 lines, so as to form a rather regular, coarsely cancellated style 

 of ornament. (Eight valve unknown.) Diameter, from the 

 hinge to the ventral margin, 0.39; breadth, from the anterior 

 to the posterior side, 0.52 inch; length of hinge, 0.58 inch; 

 convex of left valve, about 0.07 inch. 



Iu size and general appearance, this species seems to be not unlike Avicula 

 radiata of Phillips (Geol. Yorksh., ii, pi. vi, fig. i), though it has its anterior 

 wing much broader, less acutely angular, and not defined by a distinct margi- 

 nal siims. Unfortunately, however, Phillips's very brief and unsatisfactory 

 description aids but little in making the comparison. 



As we know nothing of the hinge and interior of this shell, we place it pro- 

 visionally in the genus Avicidopecten, to which it most probably belongs. 



Locality and position : Warsaw, Illinois; Keokuk group. The geological 

 position of this species was, by an oversight, given as "Warsaw Limestone," 

 in the paper cited at the head of this description. 



