THK NAUTILUS. 99 



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 line, bluish-wbite below (sometimes en- 

 tirely white) and rather thin, slightly iridescent with a wide, dark 

 prismatic border. 



Length (of type) 73.5, height 02, diam. 41 mm. 



Types (No. 33754, coll. Walker), from the Old River, Arka- 

 delphia, Arkansas. Co-types in the collections of the Carnegie 

 Museum, the Phil. Acad, of Science, the U. S. Nat. Museum, and 

 Rev. H. E. Wheeler. 



The shell characters of this fine species and most interesting ad- 

 dition to our launa 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 project- 

 ing and more anterior, and the strong, oblique folds, like those of 

 Quadrida plicata, are peculinrly 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 

 Quadrula. 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 Syniphynota. 



The smallest specimen examined (in Mr. Wlieelei's collection), 

 measures length, 35; heighth, 33, and diam. 23 mm., being almost 

 circular in shape, and looks, externally, very like a young, smooth 

 Q. pustulosa Lea, the oblique folds being only slightly evident in 

 the post-basal region. 



A very large specimen, also in Mr. Wheeler's collection, measures 

 length, 87; height, 73, and diam. 48 mm. 



The structure of the soft parts agrees entirely with that of the 

 subfamily Anodonthiae, chiefly so the mantle edge and the outer 

 marsupial gill. Unfortunately only sterile females have, as yet, 

 been obtained, but it is hoped and expected that this deficiency will 

 soon be remedied. 



We take great pleasure in naming this most interesting addition 

 to our fauna after its discoverer, the Rev. H. E. Wheeler ot Arka- 



