150 CAKBONIFEltOUS PEIMOD. 



right angles with the cardinal border ; hinge about equal in length to the lull 

 breadth of the shell ; ears prominent, posterior one most so ; anterior, basal, 

 and posterior margins regularly and continuously rounded ; beak depressed; 

 umbonal slopes moderately distinct. Left valve slightly convex or nearly 

 straight ; posterior ear about the same size as the other, forming a nearly 

 sharp angle with the cardinal extremity, its outer border being nearly 

 straight ; anterior ear triangular, well defined, its outer border slightly con- 

 vex or nearly straight, and its extremity blunt. Smface marked by ten or 

 twelve shai-ply-raised, slender, concentric ridges, each of nearly unifoi-m 

 width tlu'oughout, biit each successively a trifle larger than the preceding 

 one, separated, along the axis of the valve, by interspaces each four or five 

 times as wide as the adjacent ridges, but the interspaces diminish in width 

 toward the umbonal slopes, upon which the ridges are very near together ; 

 the latter then diverge in crossing the ears, and all end abruptly upon the cardi- 

 nal border ; smface between the ridges marked by numerous fine, uniform, 

 concentric striae, and also by very faint indications of radiating costse. 



Breadth, sixteen millimeters ; height, from base to cardinal border, 

 seventeen millimeters. 



This interesting shell is rare ; only the left valve having been dis- 

 covered. The only other locality at which it has been found, so far as I 

 am aware, is La Salle, Illinois, where the type-specimens of the species 

 were obtained, and where it occupies a geological position similar to that 

 from wMch our sp.ecimen was obtained. 



It is so unlike any other known species of Aviculopeden that no com- 

 parison is necessary. Only the exterior of the left valve of tliis species is 

 yet known, and Meek and Worthen have suggested that the undiscovered 

 parts will, if dicovered, be found to possess generic characters correlated 

 with its peculiar surface-features that will separate the shell generically 

 from Aviculopeden. 



Position and locality. — Strata of the Carboniferous period ; confluence of 

 White Mountain and Black Rivers, Arizona. 



