PALEONTOLOGY OF IOWA. 661 



Spirifer lateralis ( n. s ). 



PLATE XXIII. FIG. 7 a, b, c. 



SHELL extremely transverse, the greatest width being near- 

 ly three times the length, gibbous in the middle, gradually 

 attenuate towards the extremities. Dorsal valve regularly 

 convex, and nearly semicylindrical on the lateral portions : 

 mesial fold abruptly and strongly elevated, acutely angular 

 and much incurved towards the beak, becoming rounded 

 below. Ventral valve less convex on the sides than the op- 

 posite valve, a little more gibbous on the umbo, with the 

 beak moderately incurved : mesial sinus scarcely visible at 

 the umbo, becoming broad, shallow and scarcely defined be- 

 low : area concave, narrow, extending to the extremities of 

 the shell, and having the outer margin acutely angular. 



SURFACE marked by rounded closely arranged and little 

 elevated plications, about twenty-five of which may be 

 counted on each side, while about ten or twelve occupy the 

 mesial fold and sinus respectively ; of the latter, the six or 

 eight central ones arise from the dichotomizing of two or 

 three which originate at the beak. A large number of the 

 plications run out upon the hinge-line : minute structure 

 striato-punctate . 



In the cast, the vascular impression occupies nearly half 

 the width of the shell : muscular impressions of the dorsal 

 valve strongly marked ; margins denticulate. 



This species is so remarkably unlike any other which has fallen under my ob- 

 servation, that it may be readily distinguished. 



Fig. 7 a. Dorsal view, showing the area and foramen of the ventral valve. 



Fig. 7 b. Ventral valve of the same specimen, showing the broad undefined mesial 

 depression. 



Fig. 7 c. Front view, showing the outline and elevation of the mesial fold. 

 Geological formation and locality. In the Warsaw limestone : Clifton, 

 Illinois. 



