638 GEOLOGICAL SURVEY OF IOWA. 



Fig. 4 a. Interior of dorsal valve, from which most of the shell has been exfoliated, 

 leaving an impression of the external markings. 



Geological formation and locality. In the Keokuk limestone : Keokuk, 

 Iowa. 



Producing setigerns ( n. s.). 



PLATE XVIII. Fre.3. 



SHELL longitudinally semielliptical, as wide or wider on the 

 hinge-line than the shell below. Ventral valve gibbous, with 

 the beak much elevate \ above the hinge-line, and strongly 

 incurved. Dorsal valve very moderately concave, abruptly 

 inflected near the margin, turning upwards nearly at right 

 angles to the plane of the surface of the valve : cardinal 

 extremities auriculate. 



SURFACE marked by even rounded radiating costoa, which 

 bifurcate several times above the middle of the shell, and 

 less frequently below ; concentrically crossed by fine equal 

 undulating striae. The radiating costae become, at- intervals, 

 tuberculous, and support fine elongated tubular spines, which 

 are irregularly distributed over the entire surface of the dorsal 

 valve ; and others more closely arranged, somewhat in rows, 

 on the auriculate cardinal extremities. The cardinal mar- 

 gin of the ventral valve presents a closely arranged row 

 of tubular setiform spines, and several similar rows below 

 this on the auriculate portions of the valve ; giving a 

 compact fascicle of these appendages, which originally have 

 been nearly two inches in length, extending upwards with 

 a slight divergence, and gently curving inwards towards the 

 beak. 



This species, in its slightly concave dorsal valve, numerous setiform spines over 

 the entire surface, with fascicles of the same on the cardinal extremities, presents 

 characters by which it may be readily distinguished from any of those which have 

 fallen under my observation. The ventral valve is but partially known. 



Fig. 3. Dorsal side of specimen, showing the beak and umbo of the opposite valve, 

 with the long setiform spines from the cardinal margin. 



Geological formation and locality. In the Keokuk limestone : Keokuk, 

 Iowa. 



