SWALLOW — DESCRIPTION OF NEW FOSSILS. 93 



valve marked with a prominent trifid cardinal process, forti- 

 fied with three folds diverging from its base, one along the 

 centre towards the anterior margin, the others curving out- 

 ward and forward a little within the cardinal and lateral 

 margins, at first high and narrow, then broad and depressed 

 towards the anterior margin, where it disappears. There are 

 several irregular, curved, radiating costa3 on each side of the 

 central fold. The margin is marked with radiating costae. 



Productus Fentoxexsis. 



Shell large, hemispherical, very convex, costate. Ventral 

 valve very regularly convex ; highest near the middle; vis- 

 ceral region somewhat flattened ; anterior and lateral margins 

 form a regular semicircle: cardinal line as long as the width 

 of the shell : ears small, scarcely defined ; rounded at the 

 outer angle. Surface marked with small, fiat, radiating costa3, 

 which are obsolete on the margins, and by concentric strije, 

 and small rugae over the visceral regions. Inner surface 

 coarsely punctate and marked with radiating costae on the 

 margins. It has a few spines on the cardinal border, and 

 lines of growth on the margins. Dorsal valve not known. 



Length, 1.89; width, 2.29; height, .98. 



This fossil is very distinct from any known Productus. It 

 was collected by Dr. Shumard, at Fenton, St. Louis county, 

 Missouri. 



Productus depressus. 



Shell of medium size, depressed, semi-elliptical, regularly 

 convex, costate and corrugate. Ventral valve regularly but 

 moderately convex ; slightly depressed or sinuate in front; 

 anterior and lateral margins regularly curved into a semi- 

 ellipse : beak small: ears rather large, indistinctly defined, 

 somewhat flattened ; outer angle obtuse : cardinal line as 

 long as the width of the shell. Dorsal valve concave, closely 

 following the curvature of the opposite valve, leaving but 

 little space between them. Whole surface marked with 

 small radiating costae (increased by implantation), by unequal 

 rugae which are often most conspicuous near the margins, 

 and also by transverse striae. 



This fossil is rare in the Keokuk Limestone at Fenton, St. 

 Louis county, where it is associated with P. vittatus, P. Fen- 

 tonensis, P. (/radatus, and /S. Keokuk. 



Productus gradatus. 



Shell of medium size, broad-ovate or sub-hemispherical, 

 depressed, marked with broad, angular, spinose folds. Ven- 

 tral valve ovate, moderately convex ; highest near the beak, 



