NATURAL SCIENCES OF PHILADELPHIA. 265 



side prominent and broadly rounded ; ears unknown. Surface 

 marked by about twelve or thirteen prominent, nearly equal ribs, 

 rounded, or subangular, with broad, regularly concave interspaces, 

 wider than the ribs. Whole surface crossed by very fine lines of 

 growth. 



Length, about three inches ; greatest width 2.2 in., thickness 

 through both valves, .5 in. 



Besides the mollusca, Dr. Jos. Leidy has decided a single shark 

 tooth in the collection to belong to the common Galeocerdo fal- 

 catus of the chalk, found in England as well as America. 



EXPLANATION OF THE PLATES. 



Plate 9, fig. 1. Ammonites Guadalupse, side view. 

 " 10," 1. Id. front view, showing the change of the dorsum. Fig. let. 



Septum. 

 " 11, " 1. Lima Kimballi. 



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