120 CRETACEOUS LA.MEJ.LIBRAX('HIA. 



sliglitly curved ventral margin. Posterior margin somewhat oblique, slightly 

 concave. Postero-ventral angle rounded. Umbones high, prominent, not much 

 incurved. A jn'oiiiiuent, rounded carina extends from the umbo to the postero- 

 ventral angle, and cuts off a posterior area which is divided into two parts by a 

 sti'ong, rounded carina ending at the postcro-dorsal angle. The part of the area 

 between the carinas is concave ; the part l^ehind the median carina is deeply 

 depressed. Lunule large, very deep, flattened. Surface of shell apparently 

 smooth. Length, 48 mm. Height, 58 mm. 



Affi7iities. — It is possible that this species may be related to 0. Gallicnnd, 

 d'Orbigny,' which seems to be known by casts only, but tlie height of the shell 

 appears to be relatively less. 0. Gallieiuiei has been identified by some authors 

 with 0. hirorii!.'< (Geinitz) - f rom the Cenomanian of Plauen. The latter differs 

 from the species described above in possessing strong concentric ribs and in the 

 absence of a carina between the umbo and the postero-ventral angle. 



h'l'iniirks. — Tlie only undoubted example of this species which I have seen is a 

 right valve collected by the late W. Vicary, and now in the British Museum. The 

 surface of the shell is not well preserved, but appears to have been nearly smooth. 



An imperfect specimen of a large Opis (length about (33 mm.) from the 

 Chloritic Marl of Eggerdon Hill (Dorset) is in the Sedgwick Museum, Cambridge, 

 and may belong to this species. 



Distribtitioit. — Upper Greensand (zone of Schlfienhachhi rodnita) of Haldon. 



Oris, sp. PI. XVII, figs. 13 <i-c, 14 <i-c. 



Dcscrijitiui). — Shell small, sul)-triangular, rather oljlique ; greatest convexity 

 along the carina. Antei'ior margin rounded; posterior margin slightly convex. 

 Umbones prominent, sliarp, considerably curved anteriorly. Carina sliai'p, 

 prominent, cutting off a steeply sloping posterior area, which is not divided by a 

 median carina. Sides of sliell flattened, sloping slightly in front of the carina, and 

 passing gradually into the anterior part of the shell. Lunule very deep, with a 

 sharp border. 



Ornamentation consists of regular, concentric ribs. Postei-ior area nearly 

 smooth. 



> • Vdl. Fniiu;. 'iViT. Civt.,' vol. iii (18-J-t), pi. CL-lvii///.s tii;. 5 (not tlesfnlH.'d iu the text); 

 (ji lu'rauger, ' Alhuiii PaK'oiit. de la Sartlio ' (1867), p. 17, pi. xxii, fij,'. 13. 



2 "Das Elhthal^'el). iu Sachsen" (' Palicoiitographiea,' vol.xx. pt. 1, 1873), p. 227, pi. 1, fii^'s. 1—3. 

 luterual casts of OjJis from the base of the Chalk at Maideu Newtoii aiul Chard have beeu recorded 

 as 0. hiconm? (Gein.) by Jiikes-Browue, ' Cret. Eocks of Britain,' vol. ii (1903), pp. 113, 122. Two 

 of the specimens on which the determiuation was based are in the Oxford Museum. 



