﻿AR KAN SI A 479 



two, not strongly differentiated and coalescing along the hinge 

 line,' the anterior narrow and parallel with the hinge line, the 

 posterior somewhat wider and heavier and scarcely separated 

 from a strong projection of the interdentum, which is contin- 

 uous with the lower lateral and slopes gradually to its extrem- 

 ity ; upper lateral low, the groove between them deep and ex- 

 tending nearly to the beak ; a single, strong pseudocardinc 1 in 

 the right valve with a deep pit behind it to receive the anterior 

 pseudocardinal of the left valve, interdentum cut away to make 

 room for the interdental projection in the left valve; a single 

 strong, but rather short lateral ; ligament dark brown ; muscle 

 scars not very deep, those of the anterior adductor and pos- 

 terior adductor large, those of the anterior retractor and pedal 

 protractor rather small and inconspicuous ; cavity of the beaks 

 very deep ; nacre usually salmon-colored above the pallial Ime, 

 bluish-white below (sometimes entirely white) and rather thin,, 

 slightlv iridescent with a wide, dark prismatic border. 



Length (of type) 73.5, height 62, diam. 41 mm." (O. & W.) 

 Arkansia wheeleri Ortmann and Wai.ker, Xaut., XXV, 1912, 



p. 98, pi. VIII. 



"The shell characters of this fine species and most interestmg 

 addition to our fauna are very peculiar and can be compared 

 only with Arcidens, to which genus, Arkansia, undoubtedly, is 

 the closest affinity. In general, the external appearance is quite 

 similar, and in both the smaller series of radiating wrinkles 

 and the curved folds of the dorsal slope are very much alike. 

 But Arkansia is a much heavier and more inflated shell, with 

 the beaks fuller, more projecting and more amerior. and the 

 strong, oblique folds, like those of Quadnila pUcata, are pecu- 

 Harly its own. It differs, also, entirely in the beak sculpture, 

 which is comparatively simple and confined to the extremity 

 of the beak, and is quite similar to that of certain Quad ruler. 

 In hinge characters it differs in having strong, well-developed 

 laterals. In the presence of the interdental process in the left 

 valve and the cutting away of the interdentum in the right 

 valve opposite it, it is related to both Arcidens and Sytitphynota. 



