This is a small and slender form noted as green in life and also retaining this color after preservation in alcohol. It agrees in general with _Steggoa_, the first segment being normally developed above and distinct from the prostomium though not so clearly separated as usual, suggesting a tendency toward the _Hypoeulalia_ condition. Prostomium a little longer than wide, narrowed anteriorly, sides convex; a short lobe, rounded in front and bearing the four tentacles, is set off by a weak constriction from the basal part. Unpaired tentacle situated well caudad, more slender than the paired ones but nearly as long. Eyes not detected. Ventral tentacular cirrus of II of a thick, leaf-shaped form, sublanceolate in outline and much like the notocirri. The other tentacular cirri longer and filiform. Notocirri in outline lanceolate, characteristically exceptionally thick in proportion to width so as at times to appear nearly subconical. Neurocirri much smaller; similarly proportionately thick and at times subconical. Body slender, strongly narrowed from the middle toward both ends. The proboscis densely and uniformly papillose throughout. Number of segments near one hundred and twenty-three.
Corresponding closely in general characters with _S. macroceros_ (Grube), the genotype. Green in color instead of straw-yellow to brown. Tentacles long and slender as in _macroceros_, with the median equalling the others in length and inserted close to the base of the latter; tips of tentacles slenderly attenuated. The eyes seem to be proportionately larger than in _macroceros_. The first segment is reduced above at the sides where the prostomium bulges back on each side; but the middle region is well developed, extending forward on the base of the head as a rounded lobe or flap. Very easily distinguished from _macroceros_ and other known species by the form of the ventral tentacular cirrus of the second segment which, in place of the ordinarily lanceolate foliaceous form, is very strongly expanded above the base, presenting a large rounded lobe in front and an abruptly much more slender tip, with the blade as a whole irregularly twisted. The parapodia very similar to those of the genotype; but the setigerous lobe less acutely and less deeply notched and rather broader across the end along the setigerous line. The notocirri rather more slender and narrowed more evenly distally, not incurved on each side distally so as to leave an elongate tip set off from the rest. The neurocirri similar but more asymmetrical, the upper margin straight or concave, the lower convex. Anal cirri missing. Proboscis not protruded. Total number of segments in the type, which is complete, sixty-eight.