Mactra angusta: Deshayes' type was a young shell. It grows somewhat larger than M. californica, and is relatively a little longer and lower, with a sunken escutcheon. It gapes rather widely behind, slightly so in front. The periostracum is pale ,gray, with some brownish tint towards the lower margin, is minutely thread-striate or laminate. There are some coarse concentric wrinkles, and at the posterior irregular radial lines. The beaks have short concentric waves in front and behind. The pallial sinus is longer than in other known species of Micromactra, occupying two-thirds of the shell's length. Length 52 mm., height 30.3 mm., diam. 16 mm. As the validity of this species has been doubted by Dall, and the type was a young specimen, we are figuring a Montijo Bay shell which agrees fully with Panama topotypes.
It is readily distinguishable from M. isthmica by the narrower contour, especially posteriorly, and the somewhat longer pallial sinus, and it is at once separable from M. californica by the much longer pallial sinus. Panama (Cuming; Pilsbry, 1929). Montijo Bay (Lowe, 1931).