436 DESCRIPTION OF [^Polygastriea. 



globose forms, wliicli have a cellular testa, and in the middle of each 

 valve a coronet of hooks (aculci) or a circular membrane. 



STErHANOPYXis a<?M?e«^« = Pijxidicula aculeata (1843.) — Subglo- 

 bose, hispid. Diameter l-1440th. Virginia. 



S. Diadema. — Hemispherical, with parallel straight rows of cells ' 

 centre of disc depressed, with a circlet of closely set denticles. 13 to 14 

 cells in 1-1 152nd. Dentations 30 in the full-grown circlets. Dia- 

 meter 1 -576th. Virginia. 



Sub-genus Xajs^thioptxis, comprising those forms with two turgid 

 sub-globular valves, continuous, entu-e, and non-cellular, with a his- 

 pid, setose or winged (alate) figure. 



Thus Xanthiopyxides are setose or alate Pyxidiculce, with the habit 

 of Xanthidium, or of Chcetoti/phia, but siliceous and bivalve. 



X. alata. — Oblong, equally and widely curved on each side, and 

 smooth, with the margin of the valves uneven or deeply dentate, 

 expanded and setose. Diameter 1 -552nd. Bermuda. 



X. glohosa. — Subglobose, hispid, with short setae. Diameter 

 l-720th. Bermuda. 



X. oUonga. — Oblong, equally and widely rounded at each end, 

 surface beset with short seta), occasionally connected by intermediate 

 membranes, densely hispid. Length l-552nd. Bermuda. Dr. 

 Bailey, of New York, first pointed out and figured this species. 



X. constrida. — Oblong ; constricted at the middle, broadly rounded 

 at each end, surface hispid with short setae, occasionally conjoined 

 by membrane. Length 1 -384th. Bermuda. 



X. urccolus. — Each valve urceolate, with the summit crowned by 

 a round knob ; apertures in the revolute margin. " I (Ehrenberg) 

 have only met with single valves. In form they resemble Stephanogo- 

 nia but are not angular." Diameter 1-1 560th. Eossil in Virginia. 



Genus Biiaphoneis (E1ti\) — Lorica, of two equal quadi'angular 

 valves, navicular, not concatenate ; it is destitute as well of apertiu-es 

 and umbilicus, as of pinnules on its navicular sides, but possesses a 

 median sutural line traversing them longitudinally. 



It resembles Surirella in T\^anting the pinnules. The terminal 

 aperture at the apices of the lorica appears to be single. It differs 

 from Cocconeis and Navicula by the absence of the umbilicus. 



