26 MEMOIRS OF THE CARNEGIE MUSEUM. 



Plate II, figs. 4, 5, 6. 

 Records from Pennsylvania: 



Reported previously from Lake Erie, but not from the Pennsylvanian shores, except by Ortmann 

 (19096, p. 203) as undulala hippopma. 



Characters of the Shell: Shell of medium size, rather heavy. Outline suboval 

 to subtrapezoidal, generally slightly longer than high, somewhat oblique, rounded 

 before, subtruncate behind. Beaks moderately prominent. Beak-sculpture con- 

 sisting of three to five concentric ridges, slightly angular and nodulous behind; 

 only one to three are distinct, the others indistinct, often indicated only by the 

 nodules. The beak-sculpture is not continued upon the disk. Shell moderately 

 swollen, with indistinct, rounded posterior ridge. Surface sculptured by trans- 

 verse, oblique folds or undulations, which run parallel to each other in the direc- 

 tion of the lower posterior end. These folds are absent in very young shells, and 

 begin in older individuals at a certain distance from the beaks, and at, or a little 

 in front of, the middle of the disk, leaving the anterior part of the shell free. These 

 folds are very variable, sometimes hardly indicated, in other cases they are rather 

 strong. In large shells, there are as many as four or five of them. Posterior 

 slope comparatively narrow, since the upper posterior margin is only little, or not 

 at all elevated, so that the posterior wing is only slightly developed. In younger 

 shells, however, it is generally well-developed. The posterior slope is often entirely 

 smooth, but sometimes there are indications of radiating folds or ribs, smaller 

 than those upon the middle of the disk, and occasionally they may become rather 

 distinct and strong. In the latter case they have no distinct relation to the large 

 folds, but seem to diverge from the uppermost of the latter at an angle. Epi- 

 dermis in young specimens yellowish or greenish brown, rather light, without 

 rays. Growth-lines dark. In old shells the epidermis becomes darker, reddish 

 or chestnut-brown to blackish, and the growth-rests, which are rather regular, 

 become less marked in color. 



Hinge-teeth strong, ragged. Pseudocardinals divergent. Interdentum mod- 

 erately developed. Lateral teeth strong and rather long.- Beak-cavity of medium 

 depth. Dorsal muscle-scars on the hinge-plate. Nacre white, posteriorly often 

 very beautifully iridescent with purplish and pinkish reflections. No difference 

 whatever between the male and female shell. 



L. H. D. Pr.ct. 



Size: 1. Erie, Cat. No. 61.4.520 (largest, 9 ) 98 mm. 71 mm. 40 mm. .41 



2. do. Cat. No. 61.4,520 (cf) 68 " 54 " 30 " .44 



3. do. Cat. No. 61.4516 (cf) 46 " 36 " 20 " .43 



