502 GEOLOGICAL SURVEY OF IOWA. 



This species bears much resemblance to the Spirifer pachyrynchus of DE VERNEUIL 

 (Russia and the Ural mountains, pa. 142, pi. iii, f. 6); but the area in that species 

 is smaller and more contracted at its upper margins, particularly in old shells, which 

 are also quite free from striae. 



Fig. 1 a, b. Dorsal views of two individuals, showing the form of shell, proportion of 

 area, etc. 



Fig. 1 c, d. Ventral views of two individuals. 

 Fig. 1 e. Profile view of 1 b. 



Fig. If. Front view, the ventral valve being above. 

 Fig. 1 g, h. Dorsal and profile views of an old shell. 



Fig. 1 i. Ventral view where the shell has been ground off to show the internal spires. 

 Fig. 1 ft. Interior of the ventral valve, showing the foramen and dental lamellae which 

 extend downwards to the middle of the shell. 



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

 Hamilton group : Lime creek above Rockford, Iowa. 



Spirifer wliitneji (n.s.). 



PLATE IV. FIG. 2 a, b, c, e. 



SHELL of medium size, transverse, semielliptical or penta- 

 gonal ; height and width about as three to four. Ventral valve 

 gibbous towards the beak and curving abruptly to the base ; 

 beak elevated above ;hat of the opposite valve, and incurved 

 over the area : mesial sinus well defined at its margin by 

 stronger plications, curvilinear or subangular, beginning at 

 the beak and regularly deepening towards the base, where 

 it is often produced in a rounded or angular projection. Dor- 

 sal valve gibbous in the centre, and sloping abruptly to the 

 sides ; mesial fold elevated, prominent, rounded and well 

 defined, sometimes subangular ; beak incurved ; hinge line 

 equal to or exceeding the width of the shell : area mode- 

 rately large, concave, vertically striated ; a narrow defined 

 area on the dorsal valve : foramen broad at base and open 

 to the summit ; a narrow defined groove marks the junction 

 of the dental lamellae with the area. 



SURFACE of the mesial elevation and sinus marked by 

 dichotomizing striae, of which twelve to fifteen may be seen 

 on the margin, and three or four at the summit ; lateral por- 

 tions of the shell marked by twenty-four to thirty slender 



