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. Embouchures of rivers; China, Philippines, Borneo, India. 



FAMILY XV. MACTRIDJE. 



Shell 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 ; 

 pallial 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.} 



MACTRA, L. 



Etym. Mactra, a kneading trough. Syn. Trigonella, Da Costa (not L.) 

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



Type, M. stultorum, PI. XXI. fig. 1. 



Shell nearly equilateral ; anterior hinge tootli 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; p,alpi 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 star-fishes and 

 whelks, and in the I. of Arrau M. subtruncata is collected at low-water to 

 feed pigs. (Alder.} 



Distr. 60 sp. All seas, especially within the tropics 35 fms. 



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



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

 lage-pit simply grooved; lateral teeth very large. 



GNATHODON, Gray. 



Etym, Gnathos a jaw-bone, odous a tooth. Syn. Kangia, Desm. 



Type, G. cuneatus, PL 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 



