CRETACEOUS FOSSILS. 399 



Shell varying from medium to large size, transversely-ovate or sub- 

 elliptical in outline, usually a little larger behind than in front of the 

 middle of the length. Valves very convex, especially in the anterior half 

 and along the umbonal slope, becoming more compressed and cuneate pos- 

 teriorly, slightly unequal, the left being the largest ; beaks proportionally 

 large and slightly tumid on the umbones, but smaller and pointed toward 

 their apices, slightly incurved, and nearly terminal, somewhat elevated above 

 the hinge line, but not prominent ; hinge line long, usually nearly or quite 

 three-fourths as long as the shell below, and rounded at the extremity; 

 anterior end very short, almost on a line with the beaks and rapidly sloping 

 backwards from just below them to the basal line, giving a somewhat sub- 

 cuneate character to the anterior extremity of the shell ; posterior extremity 

 rounded ; basal line broadly rounded or forming a semi-elliptical curve, 

 being more abruptly rounded at each end than in the middle, but generally 

 slightly diverging from the cardinal line to behind the middle of the valve. 



Surface of the shell marked by numerons irregular, concentric undula- 

 tions of unequal size and strength, parallel to the margin. 



Upon one of the smaller specimens of this species (see Plate 9, Fig. 8), 

 we have been able to detect the outline of muscular scars. Those near the 

 apex of the valve are quite distinct and easily recognized, but the larger or 

 posterior scar is much more faintly marked, and we were under some doubt 

 at first if it were rightly considered as a point for muscular attachment, but, 

 after careful examination of this and a large number of individuals of dif- 

 ferent species, we think we are able to see faint traces of this larger scar, in 

 a similar position, on several of them. The anterior scar is situated quite 

 near the anterior end, and does not exceed a tenth of an inch in its greatest 

 diameter. There are five small dot-like scars between this and the apex of 

 the valve, some of them over the turn of the beak. No remains of the , 

 pallial line could be satisfactorily detected. It is probable that, like the 

 pallia! line of other shells of the Aviculidae, it was composed of isolated 

 points of attachment, and in this case would be difficult to trace, as the 

 surface of many of the casts is covered with similar points. 



In deference to the opinion of Mr. F. B. Meek, who has examined the. 

 original specimens of I. bambini, we have referred these western forms to 



