OPIS. 119 



Posterior margin truncated, slightly concave, forming an angle with the gently 

 curved ventral margin and also with the postero-dorsal margin. Umbones 

 prominent, slender, greatly incurved, almost touching in the young, Ijut separated 

 in older specimens. A prominent carina extends from the umbo to the postero- 

 ventral angle, cutting off an area which is divided into two parts by a prominent 

 but rounded carina which ends at the postero-dorsal angle ; the outer part only of 

 the area is seen in a side view, and is concave ; the inner part is depressed, and its 

 dorsal portion is flattened and resembles a lunule. Lunide very large, cordate, 

 flattened. 



Ornamentation consists of many small concentric ril)s se])arated by narrow 

 grooves. Behind and in fi'oiit of the flattened jiart of tlie valve this (irnaineiitatidii 

 becomes less distinct or may disappear altogether. Margin of valve entire. 



Measurements : 



(1) (2) (3) (4) (5) 



Length . 23 . 22 . 20 . 14 . 14 mm. 



Height' . 35 . 33 . 29 . 20-5 . l'.» „ 



Thickness . 26 . 26 . 24 . 16 . 15 „ 

 (1 — 5) Lower Greeusaucl, Upware. 



Affinities. — This species shows some resemblance to 0. Hnganliana, d'Orbigny,- 

 from the Gault. 



Eemarlis. — The shell is relatively higher in large than in small specimens. In 

 all the examples obtained from Upware the shell is preserved. Specimens irom 

 Seeud are in the Museum of Practical Geology. 



Ti/jyes. — Leymerie's specimen came from the Lower Neocomian of Bernon 

 (Aube). D'Orbigny's specimens were obtained fi'om Saint Sauveur (Yonue), 

 Bernon, and Marolles. The specimen figured by Keeping is in the Sedgwick 

 Museum. 



Dii^friliiition. — Lower Greensand of Upware and Seend. Internal casts, 

 prolnil)ly belonging to this species, are found in the Lower Greensand of Colesliill 

 near Faringdon. 



OpIS IIALDONEXSIS, sp. uov. Plate XVIII, fig. 1 ii-(J. 



DesrHjifion. — Shell trigonal, liigher tlian long, moilerately inenuilateral, convex, 

 wilh flattened sides. Anterior margin slightly convex, passing gradually into the 



' Measured from tlie umbo to the postero-veutral angle. 



i Syn. 0. SahandUina, cVOrbiguy, 'Pal. Frau^. Terr. CrOt.,' vol. iii (1844). p. 53, pi. cclvii. figs. 

 4-6 ; Pict«t auJ Roux, ' Moll. Foss. Grcs verts de Geneve' (1852), pp. 432— 434. pi. xxxii. tig. 1. 



