12 BULLETIN 122, UNITED STATES NATIONAL MUSEUM. 



median part, and this is marked by feeble lines of growth and a few 

 rather strongly impressed Avrinkles. In the interior there is a strong 

 cord over the junction of the anterior and median portion, while 

 the posterior part projects over the median part as a heavy callus, 

 having a free margin. The umbones form a strong knob from the 

 inner side of which the strong, obliquely curved sickle-like blade 

 projects through about half of the length of the shell. The ventral 

 median portions bears a strong knob. Pallets plumose, with the 

 junction of the blade and the stalk Avell marked. The blade itself is 

 lanceolate and is formed by a series of depressed cone-in-cone struc- 

 tures. The entire blade appears to be covered by a thin, horny film, 

 which bridges over the gap between the free ends of one cone to 

 the body of the next. The outer surface is well rounded; the free 

 edge of the cone forms a decidedly curved outline. The inner sur- 

 face of the blade is almost flattened, and the elements are less deeply 

 curved than on the outside. There are 17 joints to the blade of the 

 type. 



The type, Cat. No. 27415, U.S.N.M., was collected in Norfolk 

 Harbor, Virginia, and measures : Height, 8.6 mm. ; length, 8.5 mm. ; 

 diameter, 9.2 mm.; length of the blade of pallet, 9.7 mm.; greatest 

 diameter of blade, 2 mm.; length of stalk, 7.7 mm. 



We have seen this species from various stations on the Atlantic 

 coast from the Virginia Capes to the coast of Texas. 



Subgenus Nausitora Wright. 

 1865. Nausitora Wright, Trans. Linn. Soc, vol. 24. p. 451, pi. 46. 



Pallets consisting of a series of cone-in-cone elements which are 

 not entirely free at their distal ends, but fused on the exterior sur- 

 face, where some shelly material and a thick periostracum cover 

 the entire pallet. 



Type. — Bankia {Nausitora) durdopei Wright. 



KEY TO THE AMERICAN SPECIES OF NAUSITORA. 



Pallets: 



Length more than 40 mm dryas. 



Length less than 10 ram. 



Blade of pallets elongate-ovate. 



Stalk A-ery stout ftistityiila. 



Stallv slender excolpa. 



Blade of pallets not elongate-ovate. 



Blade of pallets ovate braziliensis. 



Shell : 



Height more than 18 mm dryas. 



Height less than 9 mm. 



Anterior portion of median part very broad hrasiliensis. 



Anterior portion of median part not very broad. 



Auricle large fusticula. 



Auricle small excolpa. 



