Pillai: Serpulid polychaetes from the Australian Kimberleys 
197 
25 Opercular processes consist of apparently 4, but really 3, 
dichotomously branched horns arising from a common base, two of 
them dorsolateral and the third ventral; dorsolateral horns branched 
thrice, ventral horn branched twice; opercular plate circular. Tube 
white, obtusely triangular in cross-section, with an insignificant, 
low, MLR. Spirobranchus tetraceros (Schmarda, 1861) 
-Operculum has other forms. 26 
26 Opercular process a short conical, pitted cap, arising from a cir¬ 
cular plate; distal end slanted dorsally, lacks processes . Spirobranchus sp. 2 (juveniles) 
-Operculum has other forms. 27 
27 Opercular process consists of an acute cone having a number of 
annuli; opercular plate circular . Spirobranchus acuiconus (Pillai, 1960) 
-Operculum has other forms. 28 
28 Opercular processes consist of up to about 4, of variable length, 
which may bear short lateral processes; opercular plate circular, 
flat to dome shaped. Tube light to bright pink, or pinkish red 
to brownish mauve; a very high, wavy, MLR present, more or 
less continuous in some places, interrupted or broken off in 
other places . Spirobranchus arabicus n.sp. 
-Operculum has other forms. 29 
29 Operculum lacks processes; opercular plate circular; bears a narrow, 
transparent rim . Spirobranchus sp. 3 
-Operculum has other forms. 30 
30 Operculum conical, its apex dorsally curved and bearing 
vestiges of 3 dichotomously branched horns, ending in a few 
transparent spines; opercular plate circular, flat to dome shaped. 
Tube light to bright pink or pinkish-red to brownish-mauve; a 
very high, wavy, MLR present, which projects conspicuously 
aperture; gently sloping overture; gently sloping laterally, 
bears an LLR on either side; longitudinal rows of foramina 
absent. Chaetal boss conspicuously squarish, distal part of shaft 
serrated all round . Spirobranchus elatensis n.sp. 
-Operculum has other forms. 31 
31 Operculum lacks processes; opercular plate circular, flat to concave, 
lacks processes. Tube bluish-grey to white, triangular in cross- 
section, chambered along in lateral flanges, bears a low to high 
MLR, which frequently extends as a flattened to pointed process 
over aperture . Spirobranchus kraussii (Baird, 1865) 
-Operculum has other forms. 32 
32 Operculum bears 3 main dichotomously branched, translucent 
processes, two dorsolateral and the third ventral; usually branched 
twice, sometimes dorsal processes branched thrice; a single 
medial spine located at or below second dichotomy, followed by 
1, 2, or 3 along each branch; opercular plate circular, somewhat 
dorsoventrally oval; tube unknown. Spirobranchus sp. 4 
-Operculum has other forms. 33 
