SHUMARD AND SWALLOW—NEW FOSSILS. 217 
Propuctus Boonensis, Swallow. 
_ Shell small, reie detect — without : a mesial 
bases of the spines on the dors rg car dial wiirgie slightly 
curved, as long or longer than the width of the shell; dorsal 
valve very gibbous, often varying from hemispherical to 0 ob- 
lique conical, strongly and regularly arched; slopes from the 
visceral region to the ears v very abrupt; beaks prominent, 
—— incurved, scarcely passing beyond the cardinal line; 
ge, flattened, strongly corrugated, with about seven ruge 
to shestaigiioal Pape see each fold sustaining _ 
or more spines, arranged in lines parallel to the cardinal m 
gin; outer angle sally susaente with aslight sinus beteeeen 
them and the margins. Ventral valve semi-orbicular, 
very concave, sees specger ears oe from he: vis- 
distinet concentri cae, ‘from: the P. undij 
Pena armani: bythe mann of splent on the ven- 
the more e distinct concentric ruge. 
