50 BULLETIN 71, UNITED STATES NATIONAL MUSEUM. 



perforate, smooth or ornamented; aperture distinctive, an arched 

 opening with a vertical notch or slit in the middle of the lower edge, 

 usually with tooth-like projections upward at either side. 



The aperture. is really the distinguishing character of this genus. 

 It differs much from that of the other genera of the family, and 

 there are considerable differences in the various species, some hav- 

 ing a simple vertical slit without teeth, others a broad notch with 

 a well developed tooth at each side. This wall containing the slit 

 or notch is usually well within the arched space, and is not always 

 easy to see when examining material. There are several described 



species, but none of them seem to be at 

 all common. None of the species have 

 previously been recorded from the North 

 Pacific. 



PLEUROSTOMELLA ALTERNANS Schwager. 



Plcurostomdla altcmans Schwager, Novara 

 Exped., geol. Theil, vol. 2, 1866, p. 238, 

 pi. 6, figs. 79, 80.— Terrigi, Atti. dell' 

 Accad. Pont., aim. 33, 1880, p. 199, pi. 2, 

 fig. 46.— H. B. Brady, Rep. Voy. Chal- 

 lenger, Zoology, vol. 9, 1884, p. 412, pi. 51, 

 figs. 22, 23.— Chapman, Proc. Zool. Soc. 

 London, 1895, p. 25. 



Description. — Test elongated, com- 

 pressed, very gradually tapering from the 

 subacute base; apertural end acute in 

 front view, in side view broadly rounded ; 

 chambers numerous, longer than wide ; su- 

 fig. si.-pleurostomella alter- tures slightly depressed ; aperture broadly 

 nans, x 60. a, front view; 6, arched; notch of the lower wall broad, 



VIEW SHOWING APERTURE. . . , . . 



almost semicircular, with an acutely 

 pointed tooth projecting forward on each side; wall smooth and 

 polished; color white. 



Length 0.60-0.85 mm. 



Distribution. — There is a single specimen in the collection from 

 Albatross station D2806, in 1,379 fathoms, off the Galapagos Islands. 

 It has not been met with in material from any other part of the 

 North Pacific. 



Whether or not this species is really P. alternans Schwager may be 

 open to question. That species, as the original figures show, has a 

 very high vertical wall in which the aperture is seen near the top as 

 a relatively small opening. In the recent specimens figured this 

 opening is much larger and broader. 



a 



