ARCiD-a:. 259 



undulated ; mantle ojDen, margins simple, with minute ocelli ; 

 gills equal, lips continuous with the gills. 



The teeth of Pectuuculus and Area increase in number with 

 age, by additions to each end of the hinge-line, but sometimes the 

 central teeth are obliterated b}' encroachments of the ligament. 



CNiSMA, Maj^er, Small, obliquely oval, very inequilateral, 

 margin smooth ; hinge ver}^ thick, with three anterior and four 

 posterior teeth. P. nuculatiis, Lam. Eocene. 



LiMOPSis, Sassi, 182T. 



Syn. — Pectunculina, d'Orb., 1844. / 



Distr. — 5 sp. Red Sea (Nyst.), Japan, Britain. Mr. M'Andrew 

 has dredged L. pygmaea, living, on the coast of Finmark ; it is 

 a fossil of the Pliocene of England, Belgium and Sicily. Fossil, 

 36 sp. Trias — ; United States, Europe. L. complanata , Orb. 

 (exxvii, 15). L. alter, Desh. (cxxvii, 76). L. Gyssei, Rain- 

 court (cxxx, 36, 37). 



Shell orbicular, convex, slightly oblique ; ligamental area with 

 a triangular cartilage-pit in the centre ; hinge with two equal, 

 curved series of transverse teeth. 



TRiGONOccELiA, Nyst., 1836. (Trinacria, Mayer.) Shell 

 approaches Leda in foi'm, and differs from Limopsis in the 

 absence of the expanded ligamental area. Fossil, 7 sp. 

 Eocene ; Paris Basin, Belgium, England, United States. L. 

 inequivalvis, d'Orb. (cxxvii, 77). 



CYRiLLA, A. Adams. (Huxleya, A. Adams, 1860.) Shell 

 oblong, oblique, very inequilateral, covered with a thin epidermis ; 

 hinge with six diverging, sharp teeth, directed posteriorly, and 

 terminating with a curved lamina ; ligament situated in a small 

 pit under the beak. L. sulcata, H. Ad., from the Straits of 

 Korea, dredged from 63 fathoms. 



NucuNELLA, d'Orb., 1850. 



Syn. — Nuculella, Chenu. Stalagmium, Nyst. (in part). 



Distr. — N. Nysti, Galeotti (cxxvii, 54, 55). Tertiary; Bel- 

 gium. 



• Shell suborbicular, slightly inequilateral, beaks close together ; 

 hinge-line curved, with numerous cross-pliciform teeth, inter- 

 rupted under the iDeaks by an oblique, simply granular pit, this 

 last appearing to have been for the ligament, which must at least 

 have been partially internal. 



? Cytherodon, Hall, 1873. 

 Distr. — 4 sp. Palgeozoic ; Hamilton Group, U. S. G. (Nucu- 

 lites) appressus Conr. (cxxvii, 63, 64). 



Shell subtrapezoidal, with prominent beaks and posteriorly 



