Shell elongated, conical, but less slender than I. colon, yellowish white, blunt-tipped, with eighteen or nineteen rather rounded whorls ; nucleus about twice as large as that of I. colon smooth, inflated, rounded on top, larger than the succeeding nuclear whorl, which has two inconspicuous narrow keels which are wavy and almost tuberculate from the first, and pass imperceptibly into the usual sculpture of the shell; spiral sculpture of two rows of somewhat elongated tubercles (about eighteen to the last turn), sometimes degenerating into a wavy riblet; these tubercles are arranged much as in I. colon, but are narrower in a direction transverse to the whorl, with their transverse connections less evident, the spiral ridge proportionately stronger, and the tubercles individually loss conspicuous ; the two spiral rows of tubercles, especially in the an¬terior part of the shell, occupy the peripheral third of the visible part of the whorl; the anterior and posterior thirds are somewhat excavated toward the suture, the shell being appressed and slightly raised on each side of the latter, but without forming a regular band or riblet, unless in the very last whorl where the raised edges are a little waved in sympathy with the tuberculation of the periphery ; suture very distinct; the excavation above referred to gives a particularly rounded appearance to the whorls, resulting in a wholly different aspect from that given by the subcylindrical I. colon ,which has twenty-three whorls in the same space as sixteen of Strobiligera ibex; base rounded with three strong raised threads between the anterior tubercular spiral and the canal; canal short, a little recurved at the tip ; pillar with a strong callus ; posterior canal and outer lip not completed in any of the specimens at hand, but the more adult ones indicate a rather wide roundish mouth ; outlines of the spire a little concave from the button-like nucleus. Lon. of shell, 11.0; of last whorl, 3.0; of (immature) aperture, 1.5. Max. lat. of shell, 1.87 ; of nucleus, 0.5 mm.
This shell tapers more rapidly than I. colon, as will be seen by the measurements, and differs in the other particulars mentioned from that species, which appears to be its nearest ally.

Cuba. Off Cape San Antonio, 640 fms. Yucatan Straits, 640 fms. Sigsbee, off Havana, 450 fms.