Type material. NE of Adak Island, Andreanof Island, Aleutian Islands, Alaska (51°58.50' N, 176°22.95' W), 121 m, R.N. Clark, R/V Dominator (23-971-122 = LACM 1997-152), 11 July 1997, holotype LACM 3395, 8.4 mm, lv. Paratypes: Inanudak Bay, NW side Umnak Island, Aleutian Islands, Alaska (53° 22.38' N, 168° 34.42' W - LACM 1986-336), R. Baxter, 19 September 1986, 90 m, LACM 3396 , 6.6 mm, 1 dd; N of Carlisle Island, Islands of Four Mountains, Aleutian Islands, Alaska (52° 58.53' N, 170° 02.60* W = LACM 1986-333), R. Baxter, 18 September 1986, 121 m, 1 lv, LACM 3397. Distribution. Aleutian Islands, Alaska, living at 121m.
Description. Shell small for genus, up to 8.4 mm in length (holotype), probably subadult, length/width ratio 1.8. Broadly biconical in shape, lightly built, lamellae present; subsutural ramp very narrow, weakly sloping, almost horizontal. Color light tan with lighter colored axial lamellae, edge of aperture white, light brown within. Spire moderately high with 1.5 protoconch whorls and up to 3.25 broad, convex, weakly shouldered, lamellate teleoconch whorls; suture impressed. Protoconch small, whorls rounded, smooth, maximum width 650 µm, terminal lip eroded. Axial sculpture of teleoconch whorls consisting of low, broad, close-set, numerous lamellae. First whorl partly eroded, penultimate whorl with 24-25 lamellae, last with 25-31. Spiral sculpture absent, except very subtle sculpture in P1. Aperture large, ovate; columellar lip narrow, smooth, adherent. Outer lip smooth, thin, interiorly smooth. Siphonal canal short, broad, weakly recurved dorsally, broadly open ventrally.
Remarks. Boreotrophon obesus is a very peculiar species that differs from any other Boreotrophon species in having a broad, final teleoconch whorl, a wide, very short siphonal canal, with numerous, low, broad and close-set axial lamellae.
It differs from the more southern Boreotrophon tolomius (Dall, 1919) described from off San Miguel Island, southern California, in having a comparatively higher spire, a much narrower, tabulate subsutural ramp, and a broader, much shorter siphonal canal. The three type specimens are probably immature with a maximum of 3.25 teleoconch whorls, but this species is undoubtedly separate from any other known living Boreotrophon species. This small species also differs from any other species of Buccinidae, such as Retifusus species, by its broad, short, broadly open siphonal canal and chiefly in having the typical, narrow axial lamellae of Boreotrophon.
Etymology. Obesus (L): the specific name meaning obese reflects the broad last teleoconch whorl.