LAMELLIBRANCHIA TA. 
175 
LEPTODESMA, Hall. 
The species arranged under the generic designation of Leptodesma present a 
very remarkable assemblage of forms, beginning (as here arranged) with those 
resembling Avicula and passing through various phases in form, proportions, 
etc., until the wing becomes nearly obsolete, the byssal sinus obscure, and the 
anterior end rounded, and the entire shell assuming the form of Mytilus or 
Modiola, Sanguinolites, Modiomorpha and allied genera. For convenience the 
species have been arranged into natural groups in the following order: a, spini- 
fera; b, umbonata; c, rostrata; d, patulata; e, arcoidea; f, mytiloidea. These 
relations will be more fully explained under the generic description and discus¬ 
sions in the Introduction to this volume. 
Section a, spinifera. 
Leptodesma Marcellense. 
PLATE XVII, FIG. 12. 
Leptodesma Marcellense, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 17, fig-. 12. Jan., 1883. 
Shell small, sub-rhomboidal; body obliquely ovate; height about two-thirds 
the greatest length; margin regularly rounded, extended posteriorly and 
continuing in a direct line to the beak. 
Left valve moderately convex. Right valve unknown. 
Hinge-line straight, greatly extended posteriorly, reaching beyond the 
margin of the valve. 
Beak obtuse, prominent, nearly erect, situated on the anterior third of the 
hinge. Umbonal region prominent, sloping abruptly on the posterior, not 
well defined on the anterior side. 
Anterior end limited by a shallow, obscure sulcus; margin regularly 
rounded. Wing narrow-triangular, much extended along the hinge, termin¬ 
ating in a mucronate process; margin deeply and acutely sinuate. 
