152 Trans. Acad. Sci. of St. Louis. 



ridges at right angles, and straight to the middle of the shell 

 extremities, between which the pallial margin is regularly 

 curved. Wings distinct, the anterior slightly inflated, 

 rounded at the extremity, and separated from the body of the 

 shell by a rather acute notch, from which a furrow extends 

 to the beak ; posterior wing flattened, acute, subtriangular, 

 with a shallow sinus below. Body of shell smooth ; wings 

 with radiating ribs, strongest on the anterior wing and crossed 

 by equally strong concentric lines ; posterior wing with fine 

 concentric lines." 



Length of hinge-line in the type specimen 6^ mm., height of 

 shell about 7 mm. 



Remarks. This species was originally named A. occidenialis 

 by Winchell, but that name being preoccupied, it was changed 

 to A. iowensisby Miller in his work on North American Geology 

 and Paleontology. The only individual which has been exam- 

 ined is the single type specimen in the University of Michigan 

 collection. The perfectly smooth body of the shell is the 

 chief peculiarity of the species, but this may be due to an 

 eroded condition of the type. 



Aviculopecten sp. undet. 



An imperfect specimen about 14 mm. in height has been 

 observed from this bed, which is apparently distinct from A. 

 ioivensis, but the single individual is too imperfect for identi- 

 fication or definition. 



MlCRODON LEPTOGASTER (Win.). 



PI. XII. f. s. 



Sanguinolaria leptogaster. Bull. U. S. G. S. 153: 537. 



Original Description. " Shell small, thin, subquadrangu- 



lar. Beaks subcentral, flat, not elevated above the dorsal 



line. Posterior end obliquely truncated ; anterior gently 



rounded below, abruptly above, with a long deep lunette; 



ventral side arcuate in the middle, joining the extremities by 



a Gradually increased curvature. Umbo flattened, — a low 



ridge extending obliquely to the posterior basal angle. 



Dorsal line straight behind the beaks, joining the posterior 



side at an angle of 125°. Surface marked by fine regular 



striae parallel with the ventral and posterior margins." 



