LAMELLIBEANOHIATA OF THE LOWER MARLS. 175 



ANATINIDiE. 



Genus PHOLADOMYA, Sowerby. 



Pholadomya occidentalis. 

 Plate XXIV, Figs. 1-3. 



Pholadomya occidentalis Morton. Synopsis, p. 6S^ PL VIII, Fig. 3. Gabb, Synop. Gret. 

 Form., Meek, Smithson. Inst. Checklist, etc. 



Shell of rather more than medium size, ovate in outline and very ven- 

 tricose, and as far as can be safely determined from all the specimens which 

 I have seen the valves closely meet at both extremities. Beaks large and 

 broad, incurved so as to be nearly in contact, but not prominent, resting 

 only moderately above the hinge-line, and situated well forward. Hinge- 

 line straight, nearly two-thirds as long as the shell and bounded by a rather 

 distinct and broadly excavated cardinal area, the limit of which near the 

 beaks on each side of them is distinctly angular. Anterior end of the shell 

 obliquely truncated; posterior end more narrowly rounded, and the basal 

 margin strongly curved, being a little gibbous in its curvature near the 

 middle of its length. 



Surface of the specimens, the shell never being preserved, is marked 

 by strong radiating ribs, which are most numerous and most closely arranged 

 near the middle of the valves and gradually become more distant toward 

 each extremity, those on the extreme anterior end and those near the 

 postero-cardinal border being nearly or quite twice as far apart as those on 

 the middle of the valve. On many individuals every other ray along the 

 central section of the valve, and sometimes extending well toward the pos- 

 terior, is seen to die out before reaching the beak. By this arrangement 

 the disk of the shell has been divided indistinctly into three sections, marked 

 by the simple spaces between the rays on each end, and by having inter- 

 calated rays between on the central space. On the most perfect individual 

 used, figured on the plate, which is about three and a half inches in length, 

 there are about thirty ribs determinable, and on the smaller one figured 

 only about twenty-seven can be counted. The casts also show strong con- 

 centric lines of growth, which in crossing the rays gives them a strongly 

 wrinkled or nodose and sometimes a distorted and flexuose character. 



