478 MANUAL OF THE MOLLUSCA. 
sometimes a small laminar tooth close to it; lateral teeth 
doubled in the right valve. 
Animal with the mantle open as far as the siphons, its 
margins fringed; siphons united, fringed with simple cirri, 
anal orifice with a tubular valve; foot large, lnguiform, 
heeled ; palpi triangular, long, and pointed; outer gills 
shortest. 
The Mactras inhabit sandy coasts, where they bury just 
beneath the surface ; the foot can be stretched out considerably, 
and moyed about like a finger, it is also used for leaping. 
They are eaten by the star-fishes and whelks, and in the Isle 
of Arran I. subtruncata is collected at low water to feed pigs. 
(Alder. ) 
Distribution, 125 species. All seas, especially within the 
tropics ;—35 fathoms. 
Fossil, 30 species. Lias—. United States, Europe, India. 
? Sub-genera. Sowerbya, D’Orb. Isodonta, Buy. S. crassa, 
Oxfordian, France. Cartilage-pit simply grooved ; it receives 
a tooth of the opposite valve ; lateral teeth very large. 
HARVELLA, Gray. 
Lateral teeth small; sheil cordate; thin; truncated pos- 
teriorly, and obliquely striated; hgament external, separated 
from the cartilage in the inner pit by a ridge; hinge teeth 
small, 
Sub-genus, Mactrella, Gray. Mactrinula, Gray. Shell cor- 
date, abruptly truncated behind; lateral teeth short. 
GNATHODON, Gray. 
Etymology, gnathos, a jaw-bone, odus, a tooth. 
Synonym, Rangia, Desm. 
Type, G. cuneatus, Pl. XXI., Fig. 2. 
Shell oval, ventricose; valves thick, smooth, eroded ; epider- 
mis olive; cartilage-pit central; hinge-teeth ?; laterals doubled 
in the right valve, elongated, striated transversely ; pallial sinus 
moderate. 
Animal with the mantle freely open in front; margins plain ; 
siphons short, partly united; foot very thick, tongue-shaped, 
pointed; gills unequal, the outer short and narrow; palpi 
large, triangular, pointed. 
Distribution, 1 species. New Orleans. (% other species ? Ma- 
zatlan, California; Moreton B. Australia. Petit.) 
Fossil, 3 species. Chalk—. Petersburg, Virginia. 
