Mr. John Smith, of Kilwinning, has been fortunate enough to obtain from the Lower Limestone series of Auchenmade, Ayrshire, a specimen of the left valve of Aviculopecten semicostatus, Portlock, sp., which shows the hinge plate and interior most perfectly preserved. Hitherto a study of the hinge plate of Aviculopecten has been made from specimens which were merely casts of the interior, and consequently it has been impossible to observe that portion of the hinge plate immediately interior to the umbo. The Auchenmade specimen advances our knowledge of the anatomy of the genus, and establishes a character denied for it by its author, M'Coy.