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 
