Abstract
The genus Isogramma of the Isogrammidae was first established by F. B. MEEK and A. H. WORTHEN in 1870, based upon Chon tes millepunctata MEEK and WORTHEN. The generic diagnosis presented by C. O. DUNBAR and G. E. CONDRA. is as follows: Concavo-convex or plano-convex shens of transversely semielliptical outline. The greatest width is a little in front of the hinge-line and the cardinal extremities are normally a little rounded. The width is much greater than the length of the shell. The ventral valve is very gently convex and the dorsal gently concave or plane, the general form of the shell closely resembling that of a large transverse Chonetes, with the exception than in Isogramma the cardinal areas are obsolete or very narrow.
Internally the ventral valve possesses a narrow but elongate triangular muscle platform. This structure stands a little above the level of the floor of the valve, has abruptly truncated sides an-I a surface that is near'y flat or depressed toward the mid-line. There is no foramen or deltidium and probably there are no hinge-teeth. The dorsal valve bears a small cardinal process whose base is continued forward as a low, narrow ridge to about the mid-length of the shell. Obscure traces of pallial sinuses radiate from the beak over most of the interior of this valve.
The surface of the shell is marked by fine, sharply elevated, concentric line, separated by broarler, flat in!erspaces. The shell substance is moderately thick. According to BARROIS and PAECKELMANN, there is an imperforate epidermal layer. The rest of the shell, however, is abunlantly perforated with coarse, simple, tubular punctae, which open internally and in the fossil are commonly filld with matrix.
