272 PALAEONTOLOGY. 



somewhat exceedino- the width, thougli often less; i5oint of greatest width a 

 httle below the middle of the shell, the margins nearly straight from this 

 l)()iiit to the beak above and to the front below, giving the quadrangular 

 outline. Dorsal valve ventricose, more distinctly elevated along the middle, 

 forming the proportionally narrow mesial fold, which is often more sharply 

 elevated and sometimes prolonged in front. Ventral valve marked along 

 the center with a narrow depression, corresponding to the fold on the dorsal 

 valve, but narrower, abruptly marked, and extending nearly to the beak; 

 body of the valve ventricose, especially in the upper half; beak strong, 

 sharply incurved, and slightly truncate. 



Surface of the valves marked by strong concentric lines of growth at 

 unequal distances, most numerous and crowded near the margin of the shell. 



The specimens examined are all more or less imperfect from exfoliation ; 

 consequently, the true surface-features cannot be ascertained. The species, 

 however, closely resembles specimens of A. suhquadrata from the Chester 

 limestones of Illinois and Kentucky, differing mostly in the less distinctly 

 marked and narrower mesial depression of the ventral and corresponding 

 fold of the dorsal valve. The front is also sometimes much produced, but 

 not more so than is often the case with those. The species as found at 

 Chester, Illinois, and elsewhere, is quite variable, and among twenty or 

 thirty individuals from the typical localities specimens coidd be selected 

 that would con-espond in form to any of those presented in this collection. 



Formation and locality. — In dark-colored limestone of Lower Carbonif- 

 erous age; the Wahsatch limestone, at Snowstorm Hill, near Dry Canon, 

 Oquirrh Mountains, Utah. Collected by J. E. Clayton, of Salt Lake City. 



