30 University of Michigan 



lighter shells with distinct, wavy, greenish-black rays, limited 

 to the posterior half ventrad but painting the entire length of 

 the earlier growth; left valve with two pseudocardinals, the 

 anterior almost vertical, narrow and high, with the anterior 

 surface forming an almost equilateral triangle and overhanging 

 the anterior muscle-scar ; the posterior rather heavy, pyramidal 

 and trigonal; and with two short laterals; right valve with 

 two pseudocardinals, the anterior small or vestigial and tri- 

 gonal, the posterior heavy and trigonal ; with a deep cavity for 

 the reception of the posterior tooth of the left valve, often 

 followed by a low, rounded accessory tooth, and with one short, 

 stout lateral; hinge plate moderately heavy, usually well 

 arched, and, in these specimens, extensively invaded by a ven- 

 tral proliferation of the ligamental material ; beak cavities shal- 

 low, exposing or just obscuring the irregular row of deep 

 muscle pits at their anterior ends ; anterior muscle scars deep, 

 separate, the largest almost conical with the point of the cone 

 undermining the pseudocardinals, especially in the left valve, 

 little but coarsely sculptured ; posterior scars semiconfluent, the 

 largest spatulate in shape, slightly impressed anteriad, concen- 

 trically striate and iridescent throughout; nacre white or 

 tinged with buff dorsad, thickened anteriad, iridescent poste- 

 riad; pallial line well marked, crenulate. 



The male(?) shell is moderately inflated, subelliptical, with 

 the well-rounded posterior point at about the middle of the 

 height (plate XIII, fig. 52). The female(?) shell (fig. 51 is 

 the type) is narrowly or broadly (fig. 53) ovate, much inflated 

 just posterior to the center, and with two quite well-marked, 

 but rounded, radiating swellings posteriad : one extending ven- 

 trad into the posterior point of the shell which is below the 

 middle of the height, the other reaching the ventral margin 

 about one-third of the length from the posterior end. The 

 ventral margin of the female(?) shell is almost straight to 

 quite noticeably sinuate just in front of the ventral end of the 

 anterior swelling, and the margin is also often indented between 



