THE NAUTILUS. 77 



behind the posterior ridge; ligament moderately developed, red- 

 brown; hinge strong; interdentum short and flat; in the right 

 valve there is a single, strong, high, triangular and rather smooth 

 pseudocardinal which is ragged apically, and separated from the 

 dorsal margin by a deep, narrow, straight pit for the reception 

 of the anterior pseudocardinal of the left valve and cut away be- 

 hind to accommodate the posterior pseudocardinal and a long 

 sharp curved lateral; in the left valve the pseudocardinals are 

 low, ragged, and nearly confluent, the anterior one flat, and 

 nearly parallel with the hinge line, the posterior one sharp and 

 triangular, laterals long and somewhat curved; muscle scars 

 deeply impressed, the anterior confluent, the posterior separate; 

 beak cavities deep and capacious; nacre white, sometimes faint- 

 ly salmon-tinted, irridescent posteriorly. 

 Length 67.5, height 55.5, diameter 43 mm. 

 Type locality: Cache River, Nemo, Craighead County, 

 Arkansas. 



Type in cabinet of H. E. Wheeler; co-types in collection of 

 Dr. Bryant Walker, Mr. L. S. Frierson, Alabama Museum of 

 Natural History, and Academy of Natural Sciences. 



Remarks. This species is closely related to Fusconaia undata 

 (Bar.), but is readily distinguished from it by having the ante- 

 rior portion of the umbonal area and the posterior ridge almost 

 equally inflated, thus making a remarkably wide and flattened 

 area in the middle of the shell, and by its broad, flattened and 

 incurved posterior area. It further differs from undata in not 

 having its narrow, elevated beaks, and in being without the 

 flattened area in front, which is called by Mr. Simpson the 

 "secondary lunule." It is too inflated to be confused with 

 rubiginosa (Lea) or with cerina (Con.), and too inequilateral, as 

 well as too inflated, to be taken for hebetata (Con.), from which 

 it also differs in lacking the peculiar posterior end characteristic 

 of that species. 



Dr. Walker points out that young shells of the size of the one 

 figured are rhomboid and only moderately inflated. But with 

 increasing growth the inflation of the umbonal region is rapidly 

 developed, and half-grown specimens are proportionately higher, 

 shorter and more inflated than the adults. An example at this 



