308 MANUAL OF THE MOLLUSCA. 



siphons very long, united, projecting far into the branchial cavity when 

 retracted, their ends separate and diverging ; palpi large, sickle-shaped ; gills 

 long, rounded in front, the outer shortest. 



Distr. 11 sp. Embouchui-es of rivers; China, Philippines, Borneo, India. 



FAMILY XV. Mactkid.e. 



^lie/l equivalve, trigonal, close, or slightly gaping ; ligament (cartilage) 

 internal, contained in a deep triangular pit; epidermis thick; hinge with 2 

 diverging cardinal teeth, and usually with anterior and posterior laterals ; 

 paUial sinus short, rounded. 



Animal with the mantle more or less open in front; siphonal tubes 

 united, orifices fringed ; foot compressed ; gills not prolonged into the 

 branchial siphon. 



Sections of the shell exhibit an indistinct cellular layer on the external 

 surface and a distinct inner layer of elongated cells. {Carpenter.) 



Mactka, L. 



Efyrn. Mactra, a kneading trough. Si/n. Trigonella, Da Costa (not L.) 

 Schizodesma (Spengleri), Spisula (solida), Mulinia (lateralis) Gray. 



Ti/pe, M. stultomm, PI. XXI. fig. 1. 



iS//^// nearly equilateral; anterior hinge tooth A-shaped, with 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, linguiform, heeled; palpi triangular, long and pointed; outer gills 

 shortest. 



The Mactras inhabit sandy coasts, where they bury just beneath the sur- 

 face; the foot can be stretched out considerably, and moved about like a 

 finger, it is also used for leaping. They are eaten by the stai--fishes and 

 Avhelks. and in the I. of Arrau M. suhtruncata is collected at low- water to 

 feed pigs. {Alder.) 



Distr. 60 sp. All seas, especially within the tropics ; — 35 fins. 



Fossil, 30 sp. Lias — . U. States, Europe, India. 



? Sub-genus. Sowerhja, D'Orb. S. crassa, Oxfordian, France. Carti- 

 lage-pit simply grooved ; lateral teeth very lai-ge. 



Gnathodon, Gray. 



Etym- Gnathos a jaw-bone, odous a tooth. Sijn. Eangia, Desm. 



Type, G. cuneatus, PI. XXI. fig. 2. 



Shell oval, ventricose ; valves thick, smooth, eroded ; epidermis 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 



