THE NAUTILUS. 



VOL. XXV. JANUARY, 1912. No. 9 



A NEW NORTH AMERICAN NAIAD. 



BY DR. A. E. ORTMANN AND BRYANT WALKER. 



ARKANSIA, n. g. 



Shell moderately thick, subrotund to subovate or subrhomboidal, 

 inflated, with full beaks. Disk sculptured with irregular, oblique 

 folds, which are sometimes indistinct. Beak sculpture poorly de- 

 veloped, consisting of two to three double-looped bars, the loops 

 slightly swollen or tubercular, disappearing toward the disk and not 

 continuous with the sculpture of the latter. Hinge well developed, 

 with strong pseudocardinals, a very strong interdental projection in 

 the left valve and well developed, strong, but rather short laterals. 



Soft parts: Supra-anal opening separated from the anal by a 

 mantle connection, which is somewhat variable, but rather long (a 

 little shorter to somewhat longer than the anal ; the supra-anal is 

 correspondingly longer or shorter; in one case there is a double 

 mantle connection). Mantle edges, between the anal and branchial, 

 drawn together by the gill-diaphragm but not united. Inner edge 

 of anal almost smooth, that of the branchial with papillae. 



Outer lamina of outer gills connected with mantle to its posterior 

 end, and thus the gill-diaphragm is complete. Anterior end of inner 

 gills located about midway between the posterior base of the palpi 

 and the anterior end of the outer gills. Inner lamina of inner gills 

 free from the abdominal sac, except for a short distance at the 

 anterior end. Palpi of medium size, subfalcate, their posterior mar- 

 gins connected for about one-third of their length. 



