98 THE NAUTILUS. 



Gills with well-developed septa and water-tubes. The septa 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 marsupium with slightly thickened tissue, indicating that it 

 is capable of being stretched out when gravid. 



Type : Arkansia wheeleri Ortmann and Walker. 



ARKANSIA WHEELERI, n. sp. PI. VIII. 



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 luster ; beaks very prom- 

 inent, 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 remainder of the umbonal region being entirely 

 smooth ; posterior half of the disk sculptured with irregular, oblique 

 folds, sometimes nearly obsolete, which on the dorsal slope curve 

 upwards, and in front of the posterior ridge are crossed by numerous, 

 irregular, radiating, small folds or wrinkles at right angles to the 

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

 gin nearly straight in front of the beaks, then projecting in a regular 

 curve, which continues around the basal margin until it meets the 

 posterior margin at an obtuse angle about one-third up from the 

 base ; hinge margin nearly straight ; posterior margin slightly 

 curved ; posterior ridge not prominent, usually rounded, but some- 

 times obscurely binngulate ; hinge complete ; pseudocardinals strong, 

 ragged, in the left valve, two, not strongly differentiated and coal- 

 escing 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 continuous with the lower lateral and slopes gradually to 

 its extremity ; upper lateral low, the groove between them deep and 

 extending nearly to the beak ; a single, strong pseudocardinal in the 

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

 cardinal 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 posterior adductor large, 



