PALAEONTOLOGY OF IOWA. 505 



Splitter iimtilis (n.s.). 



PLATE IV. FIG. 4 a, b, c. 



VENTRAL valve semicircular or subpentangular, gibbous 

 in the middle and towards the beak ; cardinal extremities 

 somewhat produced ; beak short, scarcely incurved ; mesial 

 sinus deep, strongly defined at the margins, curved in the 

 bottom. Area moderately high, abruptly concave, defined 

 above by a sharply angular margin ; foramen narrow, reach- 

 ing to the beak, with a pseudo-deltidium. 



SURFACE marked by twelve or more strong rounded plica- 

 tions, which rapidly decrease in size as they recede from 

 the mesial sinus ; plications crossed by fine concentric un- 

 dulating lamellae. 



This species differs from the last in the greater elevation of the beak, the shorter 

 hinge extremities, and the sinus without indications of a central fold. The character 

 of the plications, or concentric lamellae, offers no important differences from Spirifer 

 submucronatus. 



Fig. 4 a. View of the ventral valve. 



Fig. 4 b. Cardinal view, showing the area and foramen. 



Fig. 4 c. Enlargement of the surface of fig. 4 a. 



Geological formation and locality. In calcareous shales of the age of the 



Hamilton group : Independence, Iowa. 







Spirifer fimbriatus. 



PLATE IV. Fio. 5 a - e. 



D elthyris fimbriata : CONRAD, Jour. Acad. Nat. Sciences, Vol. viii, p. 263. 

 Spirifer (n. s.l) : OWEN, Report on Wisconsin, Iowa and Minnesota, pi. iii, f. 7. 



SHELL transversely subelliptical : valves often almost equal- 

 ly convex ; hinge line less than the width of the shell ; ex- 

 tremities rounded. Ventral valve transversely elliptical, with 

 the upper part and the umbo round, prominent and very 

 gibbous ; beak incurved : area high and short, the exterior 

 of the shell encroaching largely upon the space within the 

 beak ; foramen very large ; mesial sinus rounded, shallow. 

 Dorsal valve most convex in the middle ; beak and margin 

 [ IOWA SURVEY.] 64 



