156 MACTRIDiB, 



gated and rather attenuated behind; punctate-striate ; casts of 

 interior with a large umbonal impression (caused by a cartilage- 

 plate, as in Lyonsia ?) and a notch in front of it. 



Mr." Billings describes in this genus, "beneath and in front of 

 the umbo, a small aperture of a semicircular shape, which appears 

 to be the entrance to a tubular passage running backwards over 

 the transverse plate into the general cavity of the body." He 

 regards it as a byssal orifice. 



Mr. J. W. Salter referred this genus to the class Crustacea.] 



( Veneracea.) 



Family MACTRIDJE. 



Shell equivalve, trigonal, close, or slightly gaping; ligament 

 (cartilage) internal, sometimes external, contained in a deep tri- 

 angular pit ; epidermis thick ; hinge with two 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 ft'inged ; foot compressed ; gills not pro- 

 longed into the branchial siphon. 



Subfamily MAGTRINJE. 



Shell oval or subtriangular, nearly close behind ; lateral teeth 

 distinct, lamellar, well-developed. 



Mactra, Linn., 1767. 



Etym. — Mactra^ a kneading-trough. 



Distr. — 150 sp. All seas, especially within the tropics; — 35 

 fathoms. Fossil, 30 sp. Lias — ; United States, Europe, India. 

 M. turgida, Gmel. (cix, 89-91). 



Shell nearly equilateral; anterior hinge-tooth A-shaped, with 

 sometimes a small laminar tooth close to it ; lateral tooth 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 trian- 

 gular, 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 if. suhtruncata is collected at low-water to feed pigs. — 

 Alder. 



mactra (restricted). (Trigonella [Da Costa], Adams.) Car- 

 dinal teeth moderate; lateral teeth elongated, linear, subequal ; 



