164 CRETACEOUS LAMELLIBRANCHIA. 



Description. — Shell ovate, slightly inequilateral ; height greater than length, the 

 difference increasing with age, usually in the proportion of 9 : 7 or 9 : 8, but in 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. Ears unequal. 



Right 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 main rib into two equal and narrow 

 ribs ; and at the same time another similar rib may appear in the main furrow, 

 giving the appearance of numerous slender ribs. All these ribs seem to die 

 out before reaching the umbo, where the s-hell (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 strige, 

 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 

 ajDCx. Posterior ear smaller, not produced, triangular, with from seven to nine faintly 

 marked radial ribs and growth-lines (two or three being strong) ; on the ventral 

 part of the ear the fine striae 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 

 a small radial rib, which often ends at varying distances from the margin, but some- 

 times is continued, becoming stronger. Close-set radial strias, 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. Ears triangular ; the anterior larger, and with radial ribs and growth-lines ; 

 the posterior nearly smooth and with famt ribs. 



' This may, liowovcr, be due to imperfect preservatiou. 



