ARCID A. 259 
undulated; mantle open, margins simple, with minute ocelli; 
gills-equal, lips continuous with the gills. 
The teeth of Pectunculus and Arca increase in number with 
age, by additions to each end of the hinge-line, but sometimes the 
central teeth are obliterated by encroachments of the ligament. 
onisMA, Mayer. Small, obliquely oval, very inequilateral, 
margin smooth; hinge very thick, with three anterior and four 
posterior teeth. P. nuculatus, Lam. Eocene. 
Linmopsis, Sassi, 1827. 
Syn.—Pectunculina, d’Orb., 1844. 
Distr.—5 sp. Red Sea (Nyst.), Japan, Britain. Mr. M’Andrew 
has dredged L. pygmea, living, on the coast of Finmark; it is 
a fossil of the Pliocene of England, Belgium and Sicily. Fossil, 
36sp. Trias—; United States, Europe. L. complanata, Orb. 
(exxyii; 115). \- 2 alier, Desh. (exxvil; 16). ~L. Gysset, 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. 
TRIGONOCELIA, Nyst., 1836. (Trinacria, Mayer.) Shell 
approaches Leda in form, and differs from Limopsis in the 
absence of the expanded ligamental area. Fossil, 7 sp. 
Eocene; Paris Basin, Belgium, England, United States. JZ. 
inequivalvis, d’Orb. (exxvii, 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. JZ. sulcata, H. Ad., from the Straits of 
Korea, dredged from 63 fathoms. 
NvucuneE.ia, d’Orb., 1850. 
Syn.—Nuculella, Chenu. Stalagmium, Nyst. (in part). 
Disir.—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 beaks by an oblique, simply granular pit, this 
last appearing to have been for the ligament, which must at least 
have been partially internal. 
? CyrHERoDON, Hall, 1873. 
Distr.—4 sp. Paleozoic; Hamilton Group, U. 8. C. (Nucu- 
lites) appressus Conr. (cxxvii, 63, 64). 
Shell subtrapezoidal, with prominent beaks and posteriorly 
