164 CRETACEOUS LAMELLIBRANCHIA. 
Description.—Shell ovate, shehtly inequiateral ; height greater than leneth, the 
difference increasing with age, usually in the proportion of 9:7 or 9:8, but im small 
specimens of 5:4°5. Valves compressed, the right flatter than the left; antero- 
and postero-dorsal margins nearly straight, the remainder regularly rounded and 
with corrugated edges. Apical angle about 90°, but larger (sometimes 100°) in 
small specimens. Hars unequal. 
tight valve ornamented with strong, rounded, radial ribs, usually from sixteen 
to eighteen in number, but sometimes fewer or more (twelve to twenty-three). The 
ribs are separated by rather deep, rounded furrows, which are rather narrower 
than the ribs. At a distance from the margin of the valve, which varies in 
different specimens, some of the ribs are usually divided by a narrow groove, some- 
times median, sometimes on one side; towards the umbo this groove becomes 
relatively more important, and divides the maim mb into two equal and narrow 
ribs; and at the same time another similar rib may appear in the main furrow, 
ceiving the appearance of numerous slender ribs. All these ribs seem to die 
out before reaching the umbo, where the shell (to a length of about 5 mm.) 
is apparently smooth.’ At distant intervals well-marked growth-ridges are 
seen, and also (in some cases) very fine concentric lines. The surface of both 
ribs and furrows (but especially the latter) is covered by close-set radial striee, 
which at the middle of the valve are parallel with the main ribs, but, in passing 
to the anterior and posterior margins, become more and more oblique to the 
main ribs, and also more irregular and less continuous. Anterior ear large, 
produced, with a deep byssal sinus; surface with sinuous growth-lines (some 
strong), and two or three faintly marked radial ribs at the middle of the ear near its 
apex. Posterior ear smaller, not produced, triangular, with from seven to nine faintly 
marked radial ribs and growth-lines (two or three beimg strong); on the ventral 
part of the ear the fine striz of the rest of the shell are continued, and cut the 
ribs obliquely. 
Left valve with ribs similar to those of the right, but rather narrower, and 
separated by broader furrows. The ribs are sometimes divided by a small groove, 
which may reach the margin of the valve. In the main furrows there is frequently 
asmall radial rib, which often ends at varying distances from the margin, but some- 
times is continued, becoming stronger. Close-set radial striz, like those on the 
right valve, occur on both ribs and grooves. Fine concentric ribs occur in places, 
especially on the earlier parts of the valve, and also, at intervals, strong growth- 
ridges. ars triangular; the anterior larger, and with radial ribs and growth-lines ; 
the posterior nearly smooth and with faint ribs. 
1 This may, however, be due to imperfect preservation. 
