98 Trans. Acad. Sci. of St. Louis. 



forward, situated at a point about one-fifth the length of the 

 shell from the anterior end. A prominent, subangular urn- 

 bonal ridge extends from the beak to the postero-ventral 

 margin, and midway in the dorsal slope there is another sim- 

 ilar but less prominent ridge. The greatest convexity is near 

 the middle of the valve, upon the umbonal ridge. Surface 

 marked by fine, concentric lines of growth. 



Remarks. This species resembles S. rigidus but is much 

 smaller, is proportionately narrower posteriorly, and lacks the 

 strongly angular posterior margin. It is possible that this 

 species is but a small or immature form of S. rigidus, but the 

 specimen illustrated seems to be an adult shell. The speci- 

 mens indicated as " types " of this species in the University 

 of Michigan collection, are four in number, the best preserved 

 of which is here illustrated. 



Sphenotus iowensis (Win..) 



Pi. iv. f. 7. 



Sanguinolites iowensis. Bull. U. S. G. S. No. 153: 538. 



Shell subelliptical in outline, length, 26 mm., height, 

 13 mm. Hinge-line arcuate, the dorsal margin sharply in- 

 flected to form a long cartilage groove; anterior margin 

 sharply rounded above to the anterior extremity of the hinge- 

 line under the beak, regularly rounded below ; ventral margin 

 gently curved; posterior margin short, truncate, meeting the 

 ventral margin in a sharply rounded angle, and the dorsal 

 margin in an obtuse angle. Beaks elevated above the hinge- 

 line, somewhat flattened, incurved and directed forward over 

 a deep lunette. A prominent, sharply angular or carinate, 

 slightly sigmoid umbonal ridge extends from the beak to the 

 postero-ventral angle, and a still sharper ridge along the 

 dorsal margin of the shell bounding the long cartilage groove; 

 the twisted, flattened triangular space between these two 

 ridges is marked by three faint, depressed lines radiating 

 from the beak. Surface marked by irregular lines of growth 

 which are most conspicuous on the anterior portion and faintest 

 on the postero-dorsal slope. 



Remarks. The above description is based upon the speci- 

 men of the species marked " type " in the University of Mich- 



