LAMELLIBRANCHIATA. 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 

 antei-ior 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 l)e more fully explained under the generic description and discus- 

 sions in tlie Inti'oduction to this volume. 



Section a, spinifera. 



Leptodesma Marcellense. 

 PLATE XVII, fk;. :-2. 



Li-ptodesnia Mai-celleuse, Rxhh. Pal. N. Y., vdl. v, pt. 1. Pl.-ites and Exiilanations : Pl. 17, tii,>-. 12. Jan., 1883. 



Shell small, sub-rhomboidal ; )»ody 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 jjrominent, 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. 



