PELEOYPODA. 49 



attenuated. The beaks are slightly inflated, depressed, and situated in the anterior 

 half of the shell. The anterior margin is broadly rounded and passes gradually 

 into the almost straight ventral margin. The posterior margin is short and straight 

 and forms an angle of nearly 90° with the ventral margin. The valves are moder- 

 ately inflated, and gradually slope towards the ventral margin. A rounded keel, 

 which runs from the umbo to the corner of the central and posterior margin, 

 divides off a gradually sloping posterior area ; a second more obtuse keel, running 

 from the umbo to the corner of the posterior margin, limits a long and deep liga- 

 mental area. Lunula longer than broad, narrow. Coarse, irregular, concentric 

 striae of growth form the only ornamentation of the surface. 



Locality and stratigraphical position. — Mazdr Drik, horizon 8. 



Bemarks. — The general shape of the shell is more like that of a Crassatella 

 than that of a Cyprina. The question of the generic position of this species can only 

 be settled satisfactorily when the characters of the hinge are known. It was 

 chiefly the deep groove behind the beaks, probably containiog the external liga- 

 ment and the apparently non-crenulated ventral margin which led me to assume 

 that this species belongs to the genus Cyprina, Although closely related in regard to 

 the shape of the shell to Cyprina mazariana, it is, however, easily distinguished from 

 it by its more transversely elongated valves. Another distinguishing feature is the 

 more gradually sloping posterior area of Cyprina desensis. 



From Cyprina forhesiana it is easily distinguished by its flatter, less inflated 

 valves, the more median position of the beaks, a narrow but well defined lunula, 

 and a long concave area, just above the posterior hinge margin of the valve in 

 which the ligament is situated. 



Genus: EOUDAIRIA, Mun. Chalm. 

 RoUDAiKiA. CEA.SSOPLICATA, spec. nov. PI. XII. fig. 8-8a. 



Unfortunately only a right valve, which is partly covered with young shells of 

 species of Ostrea, has come under examination. The shell is trigonal in shape and 

 strongly inflated. The umbo is prominent, attenuated and strongly curved for- 

 wards and inwards. A strong and sharp keel runs from the apex to the junction of 

 the posterior and ventral margin. In front of this keel the surface slopes moderately, 

 while behind it the shell drops very suddenly. There is a second keel on this pos- 

 terior one, running from the apex to the corner of the posterior and cardinal margin, 

 in front of which the surface is deeply grooved. Behind the keel the surface of the 

 shell is smooth, except for some striae of growth, but in front of it the ornamenta- 

 tion is quite peculiar. Near the umbo it consists of broad and sharp concentric 

 ribs, separated by broad concave interstices. With increasing size the ribs become 

 pronounced and sharp, and the interstices grow broader and angular, being divided 

 into a narrow and steep dorsal, and a broad, but slightly sloping ventral part. 

 The surface of the shell resembles therefore a flight of low, broad stairs, 

 gradually narrowing from the ventral margin to the apex. 



