232 Observations upon the Geology and Paleontology 



incurved, leaving but little space between the two beaks. 

 Width and height of foramen about equal, nearly filled by 

 the strong cardinal process. 



Muscular cavity large, heart-shaped, with a forked sep- 

 tum occupying its centre. Surface marked by fine striae, 

 with occasional tubular openings on the surface. Strias 

 increasing by implantation, and gradually bending out 

 toward the lateral margins. 



Imbricating lines usually confined to a space near the 

 margins. This shell is variable in the convexity of the 

 dorsal valve, the distinctness of the dorsal sinus, and the 

 strength of the cardinal and brachial processes. Width a 

 little more than the length, usually less than half an inch. 



In its details, this species considerably resembles the 

 western forms of Orthis Vanuxemi of the Hamilton 

 group, but, beside other differences, it is usually of 

 smaller size, more gibbous, and the ventral beak is more 

 elevated and incurved. 



Geological formation and locality. In beds Nos. 1 to 

 the lower portion of No. 7, inclusive, Burlington, Iowa. 

 Dedicated to Dr. Otto Thieme, of Burlington, Iowa. 



SPIRIFER SOLIDIROSTRIS (White.) 



Shell rather small, nearly semicircular, wider than long, 

 widest at the hinge-line, where it is sometimes extended 

 into submucronate points, rounded in front. 



Dorsal valve more convex from beak to front than trans- 

 versely. Beak scarcely prominent, slightly projecting be- 

 yond the hinge-line. 



Ventral valve about twice as deep as the opposite one, 

 regularly arcuate from beak to front, but a little depressed 

 near the cardinal extremities. Area large and well de- 

 fined, foramen narrow, beak acute, incurved, and becoming 

 solidified as the foramen is progressively closed. Dental 



