﻿47*^ ARKANSrA 



( )utcr lamina of outer gills connected w itli mantle to its pos- 

 terior end, and tlms the gill-diaphrai^^m is complete. Anterior 

 end of inner gills located about midway between the posterior 

 base of the palpi and the anterior em\ of the outer gills. Inner 

 lamina of inner gills free from the abdominal sac, except for a 

 short distance at the anterior end. I'alpi of medium size, sub- 

 falcate, their j^ostcrior margins connected for about one-thi xl 

 of their length. 



Gills with well-developed septa and water-tubes. 'J'he scjjia 

 are rather distant in the male and in the inner gill of the female. 

 The outer gill alone is marsupial in the female, with very close 

 septa. Edge of marsuj)ium with slightly thickened tissue, indi- 

 cating that it is capable of being stretched out when gravid. 



Type, Arkansia whcclcri <^ )rtmann and Walker. 



Arkansia wnEiCLKKi Ortmanu and Walker. 



"Male and female .shells alike. Shell subrotund to subovate 

 or subrhomboidal, inflated, rather thick and solid ; dark reddish- 

 brown or black, usually lighter toward the beaks, which in 

 young shells are chestnut-colored ; epidermis with a silky lus- 

 ter ; beaks very prominent, projecting anteriorly and incurved 

 over the large lunule, their sculpture consisting of two or three 

 double-looped bars, the loops slightly swollen or tubercular ; 

 sculpture restricted to the extremity of the beaks, the remain- 

 der of the umbonal region being entirely smooth ; posterior half 

 of the disk sculptured with irregular, oblique folds, sometimes 

 ncarl}' obsolete, which on the dorsal slope curve upwards., and 

 in front of the posterior ridge are crossed by numerous, irreg- 

 ular, radiating, small folds or wrinkles at right angle; to the 

 lines of growth ; anterior portion of the disk smooth ; anterior 

 margin nearlv straight in front of the lieaks. then projecting 

 in a regular curve, which continues around the basal margin 

 until it meets the ])osterior margin at an obtuse angle about 

 one-third up from the base; hinge margin nearly straight: pos- 

 terior margin slightly curved ; posterior ridge not prominent, 

 usually rounded, but sometimes obscurely biangulate : hinge 

 complete; pseudocardinals strong, ragged, in the left valve. 



