PALEONTOLOGY OF IOWA. 643 



the mesial fold : those on the sides are nearly always simple, 

 while some of those in the sinus bifurcate, and those on the 

 fold are formed by the bifurcation of two simple ones at the 

 apex, and are sometimes smaller than the others. 



SURFACE marked by extremely fine regular radiating strise, 

 which cover the plications, and are crossed by equally fine 

 concentric strise and coarser lines of growth. 



This species has generally been referred to S. striatus, var. attenuatus ; but presents 

 well-marked differences, being much more ventricose and gibbous at the beaks, the 

 sinus and mesial fold smaller, and not more than half as many plications. I have 

 observed no intermediate forms or gradations between the present species and the 

 large one usually considered as identical with S. striatus. 



Fig. 3 a. Dorsal view, showing the area and foramen of the opposite valve. 

 Fig. 3 b. Ventral view of a larger individual. 



Fig. 3 c. Profile of a very gibbous specimen, where the beak of the dorsal valve is closed 

 beneath that of the opposite valve. 



Fig. 3 d. Profile view of the specimen fig. 3 a. 



Fig. 2 d. Enlargement of surface, showing the radiating strise. 



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

 Iowa ; Nauvoo, Warsaw, etc. Illinois. 



Spirifer neglectus ( n. s.) 



PLATE XX. FIG. 5. 



SHELL transversely oval, gibbous ; hinge-line less than the 

 greatest width, rounded at the extremities. Dorsal valve 

 convex ; mesial fold small at the beak, increasing rapidly 

 towards the front, where it is very prominent, not plicated ; 

 beak a little elevated above the hinge, incurved. Ventral 

 valve a little the more convex, elevated in the umbonial 

 region : mesial sinus broad and deep, with one broad faint 

 plication in the middle, and indications of another on each 

 side : umbo very gibbous ; beak strongly arched ; area high 

 arcuate, its lateral margins rounding gradually on each side ; 

 foramen large, triangular, a little higher than wide. 



SURFACE marked by about six broad depressed and rounded 

 simple plications on each side of the mesial fold and sinus ; 

 concentrically crossed by fine undulating striae and a few 

 stronger wrinkles of growth. 



