478 MANUAL OF THE MOLLUSCA. 



sometimes a sm.all laminar tooth close to it ; lateral tec tb 

 doubled in tlie right valve. 



Animal with the mantle open as far as the siphons, its 

 margins fringed ; sijDhons united, fringed with simple cirri, 

 anal orifice with a tubular valve; foot large, linguiform, 

 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 moved 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 M. subtruncata is collected at low water to feed pigs. 

 (Alder.) 



Distribution, 125 species. All seas, esi^ecially within the 

 tropics ; — 35 fathoms. 



Fossil, 30 species. Lias — . United States, Europe, India. 



? Sub-genera. Sowerbya, D'Orb. Isodonta, Buv. 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 ; shell cordate ; thin ; truncated pos- 

 teriorly, and obliquely striated; ligament external, separated 

 from the cartilage in the inner pit by a ridge; hinge teeth 

 small. 



Sub-genus, Madrella, Gray. Mactrinula, Gray. Shell cor- 

 date, abruptly truncated behind ; lateral teeth short. 



Gnathodon, Gray. 



Etymology, gnathos, a jaw-bone, odus, a tooth. 



Synonym, Eangia, Desm. 



Tijpe, G. cuneatus, PL XXI., Fig. 2. 



Shell oval, ventricose ; valves thick, smooth, eroded ; epider- 

 mis olive ; cartilage-pit central ; hinge-teeth f ; 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, 

 j)ointed ; gills unequal, the outer short and narrow ; palpi 

 large, triangular, pointed. 



Distribution, 1 species. New Orleans. (3 other species ? Ma- 

 zatlan, California; Moreton B. Australia. Petit.) 



Fossil f 3 species. Chalk — . Petersburg, Virginia. 



