FAMILY MACTRIDA: — MACTRA. 231 
M. nucleus. (Conran, Ac. Sc. Vol. 6, p. 258, pl. 11; Am. Conch. pl. 14.) Small, triangular, thick, 
with an obsolete concentric ridge or angle: umbones flattened and rectilinear; apices nearly central 
and very acute; posterior slope depressed ; lateral teeth strong. Color, pale brown, New-Jersey, 
and undoubtedly on our own coast, although not yet observed. 
GENUS MESODESMA. Deshayes. 
Animal with the mantle united on the posterior two-thirds of its length, and provided on its 
posterior extremity with two short tubes, prolonged within by a very delicate membrane. 
Foot much flattened, quadrangular, partly coneealed by the gills ; these latter short, truncated 
and connected together, the external pair smallest and subauriculated. Shell solid, sub- 
trigonal, compressed and generally closed: hinge with a spoon-shaped cavity in each valve 
for the cartilage, and a simple and oblong tooth on each side. 
MeEsoDESMA ARCTATA. 
PLATE XXIX. FIG. 288, a. 8. 
(STATE COLLECTION.) 
Macetra arctata. Conran, Jour. Acad. Nat. Sciences, Vol. 6, p. 257, pl. 11, fig. 1. 
M. deaurata. Ip. Am. Mar. Conchology, p. 59, pl. 14, fig. 1. 
Mesodesma arctata. Govu.p, Invertebrata of Mass, p. 57, fig. 39. 
Description. Shell solid, subtriangular, very inequilateral ; the anterior margin short, trun- 
cated. Beaks little elevated, quite in front, with a prominent ridge to the lower angle ; pos- 
terior end produced, with the margin rounded. Surface with concentric ridges, caused by 
the different stages of growth: cartilage-pit very deep and triangular. Lateral teeth elon- 
gated, and crossed by regular elevated striz. Interior smooth; the sinus of the palleal im- 
pression orbicular, and somewhat larger than its contiguous posterior muscular impression. 
Color. Epidermis olive-yellow, with a metallic lustre ; within whitish. 
Vertical axis, 1:0; transverse ditto, k’4. Diameter, 0°5. 
This is not a very common species on our shores, but appears to be more abundant on the 
shores of Massachusetts. 
(EXTRA-LIMITAL.) 
M. jauresii. (Guerin, Mag. de Zool. 1834. Gourn, loc. cit. fig. 38.) Shell ovate, triangular, thick, 
and very rough externally with coarse concentric ridges; beaks little elevated; lateral teeth very 
strong, curved and very faintly striated. Color: epidermis dusky brown. Vertical axis, 1+1; 
transverse ditto, 1°75. Grand Banks. 
