92 TRANS. OF THE ACAD. OF SCIENCE. 



to the anterior margin ; flattened or slightly depressed from 

 the middle towards the beak: ears very large, flattened, 

 slightly arched along the cardinal margin : beak small, strongly 

 incurved over the cardinal line, which is as long as the width 

 of the shell. Dorsal valve very concave, closely following 

 the curvatures of the opposite valve ; flattened on the visceral 

 region; depressed under the beak: ears well defined and 

 flattened. Surface ornamented with small, round, depressed, 

 crowded, radiating costse (increased by implantations and by 

 subdivisions) ; it also has fine raised concentric lines, which 

 are most conspicuous on the costae ; space between the costae 

 narrow; but when exfoliated the costse become narrower and 

 the spaces between wider ; the costse often curved and geni- 

 culated : ears of the ventral valve marked Avith large, irregu- 

 lar rugae, which extend up on to the sides of the valve ; some- 

 what obsolete on the visceral region; ears and visceral region 

 of the dorsal valve marked with unequal rugae. It also has 

 a row of spines on the cardinal margin. 



Length, 3.50 ; width, 2.60 ; height, 1.35. 

 Variety magistos is larger, wider, and more depressed; the 

 costse striate longitudinally when exfoliated. 



Length, 4.00 ; width, 4.00 ; height, 1.30. 



Variety bombex smaller, more convex, more strongly 

 arched ; whole surface marked with distant, diagonal rows of 

 spines. 



This fossil most resembles the P. mquicostatus of Dr. Shu- 

 mard ; but it is much larger, longer in proportion, and very 

 differently marked. 



These magnificent fossils are abundant in the Limestones of 

 the Upper Coal Measures of Harrison county, Missouri. 



Productus auriculatus. 



Shell of medium size, depressed, semi-orbicular, costatc and 

 striate. Ventral valve most abruptly arched near the mar- 

 gins ; highest near the middle : umbo small : beak small, 

 strongly incurved to the cardinal line : ears large, flat or 

 slightly arched; recurved towards the centre of the valve: 

 cardinal line long, extremities arched back and down. Dor- 

 sal valve concave, closely following the curvatures of the Neu- 

 tral valve ; flattened or slightly concave in the middle ; de- 

 pressed under the beak ; abruptly arched upwards in front 

 and toward the lateral margins; ears well defined and flat. 

 Surface marked with equal, small costae (increased by im- 

 plant at ion), by line concentric stria", and by a few obsolete 

 transverse rugae on the visceral region, which are more con- 

 spicuous on the ears, by a few lines of growth near the mar- 

 gin, and by a row of spines on tin' cardinal margin, twelve or 

 fourteen on each side of the beak ; inner surface of the dorsal 



