Shell medium in size, from 35 to 60 mm. (1,5 to 2,5 inches) in length, rather thin and strongly spinose. Whorls from seven to nine, rather strongly convex with occasional specimens moderately carinated. Color uniformly milky white or light gray. Spire extended. Suture irregular and rather deeply impressed. Aperture obliquely ovate and porcellaneous white. Parietal lip slightly reflected, rigidly attached above, erect and free below. Generally it is smooth within though occasionally two or three very weak denticles may be formed near the lower part. Palatal lip thin, erect, somewhat irregular and not crenulated; opposite the large shoulder spine it is slightly drawn out to form a tooth-like process. The siphonal canal is extended, narrow throughout its length and rather abruptly turned upward at its base, forming a pronounced angle at the base of the aperture. In addition, occasional specimens have the canal turned slightly to the right in an apertural view. Two previous stages of the siphonal canal remain as scale-like spines at the lower left of the aperture. Axial sculpture formed by three prominent equidistant varices. The varices of each whorl arc aligned with those of the previous whorl, each varix supporting a scries of recurved spines, the largest being developed at the shoulder of the whorl. The remaining spines are quite irregular and on a few specimens, they are alternately large and very small. The spines are closed on the aperture side, though occasionally there is a narrow slit remaining. In between the varices there are two to four-nodulose axial ridges, the nodules being in both axial and spiral arrangement. Growth lines consist of very tine axial threads. Spiral sculpture consists of numerous fine cords. It is on these cords that the nodules are generally developed forming the axial ridges. Nuclear whorls two. bulbous, without a carina, smooth, with the first whorl larger. This whorl develops in a vertical plane, then turns at the beginning of the second whorl to a horizontal plane. The following whorl has spiral and axial ridges, the varices beginning at the end of this whorl. Length 65 mm,width 24mm.

S. aguayoi appears to be nearest in its relationship to M. antillarum (= Siratus formosus) but it differs by possessing a narrower and a more recurved siphonal canal, a more extended spire and having the spines on the varices far more strongly recurved. Both the parietal and palatal lips in S. aguayoi are much smoother, the parietal denticles being relatively insignificant and the crenulations on the outer lip almost obsolete. The most significant difference is that of the nuclear whorls: S. aguayoi possesses a bulbous and twisted first whorl which is larger than the second whorl. In M. antillarum the first nuclear whorl is smaller than the second and is not at all bulbous or twisted in a horizontal plane.

In deep water, the Bahamas and south to Cuba.