496 MANUAL OF THE MOLLUSCA. 
with a semicircular row of transverse teeth; adductors sub- 
equal; pallial line simple; margins crenated inside. 
Animal with a large crescent-shaped foot, margins of the 
sole undulated; mantle open, margins simple, with minute 
ocelli; gills equal, ips continuous with the gills. 
Distribution, 58 species. West Indies, Britain, India, New 
Zealand, West America; ranging from 8 to 60, rarely 120 
fathoms. 
Fossil, 80 species. Neocomian—. United States, Europe 
South India. 
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 liga- 
ment. 
Limopsis, Sassi, 1827. 
Type, L. aurita, Pl. XVII., Fig. 17. 
Synonym, Trigonoceelia, Nyst. Pectunculina, D’Orb. 
Shell orbicular, convex, slightly oblique; ligamental area 
with a triangular cartilage-pit in the centre; hinge with 2 
equal, curved series of transverse teeth. 
Distribution, 4 species. Red Sea (Nyst.), Japan, Britain. 
Mr. M‘Andrew has dredged ZL. pygmea, living, on the coast 
of Finmark; it isa fossil of the Pliocene of England, Belgium, 
and Sicily. 
Fessil, 36 species. Bath-oolite—. United States Europe. 
Nucuna, Lam. 
Etymology, diminutive of nux, a nut. 
Example, N. Cobboldie, Pl. XVIT., Fig. 18. 
Shell trigonal, with the umbones turned towards the short 
posterior side; smooth or sculptured, epidermis olive, interior 
pearly, margins crenulated; hinge with prominent internal 
cartilage-pit, and a series of sharp teeth on each side; pallial 
line simple. 
Animal with the mantle open, its margins plain ; foot large, 
deeply fissured in front, formimg when expanded a disk with 
serrated margins; mouth and lips minute, palpi very large, 
rounded, strongly plaited inside and furnished with a long con- 
voluted appendage; gills small, plume-like, united behind the 
foot to the branchial septum. 
The Nucula uses its foot for burrowing, and Professor Forbes 
has seen it creep up the side of a glass of sea-water. The labial 
appendages protrude from the shell at the same time with the 
