SABELLIDES AND SERPULIDES 237 



lery and Mesnil 1897). Superior thoracic set^e usually differing in 

 form, those of the first or collar fascicle varying from those having 

 simple tapered blades to others having a conspicuous, fin-like basal ex- 

 pansion. 



Uncini with rather numerous equal minute teeth in 2 or 3 ( ?) rows. 

 See also p. 252. 



SPIRORBIS SEMIDENTATUS sp. nov. 



p1, XXVII, figs. 7, 10; p1. XLI, figs. 13, 17, 23, 26-30; Pi. XLHI, figs. 4, 5, 12. 



Type locality. — Dutch Harbor, Unalaska Island. 



Tube thick and massive, vitreous, rarely showing any transparency, 

 opaque with dull surface, dcxtral, the few whorls not regularly 

 rounded nor spreading, but piled one above the other, forming a high 

 spire with nearly perpendicular sides and flattened top, without central 

 depression, often with a distinct angular shoulder. Aperture very 

 lustrous within, with a small round opening, the thick shell forming a 

 broad, straight, flattened, inner or columellar margin with a con- 

 spicuous projection at its junction with the thinner straight, rounded 

 top edge, from which it arches forward and spreads out in a shining, 

 somewhat iridescent layer on the body of the shell ; in some speci- 

 mens a spiral ridge appears to arise from the outer margin, and 

 is at first ill-defined, but increasing abruptly forms a conspicuous 

 keel, which ends at the aperture in an angular projection ; in such 

 instances an added prominence is given to the columellar projec- 

 tion, giving to the aperture a two-toothed appearance. The un- 

 keeled form strongly resembles S. vitreus Fabr., but forms a much 

 higher spire and has never been seen so glassy and transparent as 

 specimens of the latter from the Atlantic ; immature examples are 

 semitransparent. The carinated form is similar to S. violaceus^ but is 

 not so regularly coiled nor so deeply grooved. Others are like some 

 forms of S. variabilis., but coil in the opposite direction. 



Diameter 3 to 4 mm. ; height the same. 



Animal with 3 thoracic and about 30 posterior segments. Thoracic 

 membrane very conspicuous, partially covering the 7 branchias and 

 operculum, which expands from the stout peduncle into a cup-shaped 

 organ the size of the rounded aperture, protected by a moderately 

 thick, saucer-shaped, calcareous plate with an irregularly thickened 

 inner basal ridge ; it seems to be covered by a very thin membrane, to 

 which minute protozoans are often attached ; the edge of the oper- 

 culum appears as a dark brown rim. 



