148 CARBONIFEROUS PERIOD. 



thence in almost a direct line about half way to the cardinal margin ; thence 

 curving outward, it forms with the last-named margin a somewhat acute 

 angle. 



Hinge-line a little longer than the full breadth of the body of the shell 

 and forming right angles with its vertical axis, extending farther backward 

 than the posterior border below it does, but not reaching quite so far forward 

 as the greatest convexity of the anterior border. 



Left valve moderately convex, the greatest convexity being in the 

 umbonal region ; beak prominent and projecting a little beyond the cardinal 

 border; posterior ear moderately large, acutely angular at the outer 

 extremity, not distinctly defined from the body of the valve "by an auricular 

 furrow; anterior ear defined by a moderately deep byssal sinus and a dis- 

 tinct auricular fun-ow, not so prominent as the other ear ; its outer border 

 rounding downward and inward from the cardinal border into the byssal 

 sinus, where it is met by the incurving anterior border of the body of the 

 valve. Surface marked by numerous fine, radiating costse of unequal size, 

 which are in tiu'n marked by very fine, radiating stiise, all of which are 

 crossed by fine concentric lines of growth and occasional coarser lines of 

 increment ; upon the posterior ear, the radiating costte are obsolete, but 

 upon the anterior ear they are coarser than those of the body of the valve, 

 and are also somewhat corrugated. Right valve unknown. 



So far as the rather numerous, but more or less broken, examples of 

 this shell will permit determination, the height, breadth, and length of the 

 hinge-line are all nearly equal, being about six centimeters for the largest 

 example. 



This species somewhat resembles A. occidentcdis Shumard, which is 

 found in rocks of the same period, and perhaps associated with it ; but it 

 differs from that species in its greater proportionate breadth, in being less 

 contracted below the ears, its less distinctly- defined posterior ear, its finer 

 and rather more unequal costce, and greater size. 



Position and locality. — Strata of the Carboniferous period ; Bear Spring, 

 near Camp Wingate, New Mexico. 



