156 MACTRIDA. 
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.) 
Famity MACTRID. 
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 fringed ; foot compressed; gills not pro- 
longed into the branchial siphon. 
SuspraAMity MACTRINA. 
Shell oval or subtriangular, nearly close behind; lateral teeth 
distinct, lamellar, well-developed. 
Maocrra, 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 M. subtruncata is collected at low-water to feed pigs.— 
ALDER. 
MACTRA (restricted). (Trigonella [Da Costa], Adams.) Car- 
dinal teeth moderate; lateral teeth elongated, linear, subequal ; 
