520 GEOLOGICAL SURVEY OF IOWA. 



Spirifer extenuatiis (n.s.). 



PLATE VII. FIG. 6. 



SHELL in form of a segment of a circle ; width more than 

 two and a half times the length. Dorsal valve gibbous in the 

 middle, flattened at the sides, and much extended on the 

 hinge line ; mesial lobe prominent, rounde^ above, well de- 

 nned, free from plications, deeply emarginate in front. 



SURFACE marked by about twenty-two or more small sim- 

 ple rounded plications on either side of the centre. 



The specimen described is the cast of a dorsal valve, showing the remains of 

 closely arranged concentric strise and apparent granulations. The cardinal extremi- 

 ties are much extended in mucronate points, which, in the whole shell, would make 

 the width equal to three times the length. 



Fig. 6. Dorsal valve of this species. 



Geological formation and locality. In yellow sandstone of the age of the 

 Chemung groiip of New-York : Burlington, Iowa. 



Spirifer capax (n.s.). 



PLATE VII. FIG. 7 a - d. 



Spirifer euruteines, in part : OWEIT, Report on Wisconsin, Iowa and Minnesota, pa. 586, 



pi. iii, f. 2, 2 a. 



SHELL subglobose or subhexahedral, very gibbous ; extre- 

 mities usually salient, sometimes rounded. Ventral valve 

 subpyramidal, very gibbous towards the umbo ; beak slightly 

 incurved, abruptly sloping to the base ; sinus strongly de- 

 fined, angular above, becoming broad and curvilinear below, 

 sometimes flattened in the bottom : area forming a broad 

 triangular space ; foramen large, reaching to the apex of the 

 shell, the height equal to once and a half the width of the 

 base. Dorsal valve very gibbous in the middle, sloping ab- 

 ruptly towards the base, and flattened towards the hinge 

 extremities ; mesial fold strongly denned, rapidly expanding 

 towards the base, plain or without plications. 



SURFACE marked by sixteen to twenty simple rounded pli- 

 cations on each side of the mesial fold and sinus. The finer 

 surface markings unknown. 



