Petta Malmgren 1866

(After Zhang et al. 2019). Unique characters Petta differs from other pectinariid genera in having a pair of dorso-lateral pads on segment 5, neurochaetae present on segment 21, uncini with a rounded anterior peg and a longitudinal row of two major uncinal teeth, large basal hump on branchiae, six pairs of distinct triangular lobes on lateral margins of scaphe, and a vestigial anal flap. Petta differs from Lagis which has the cephalic veil partially or completely attached to the operculum, whereas the cephalic veil of Petta is completely free from the operculum. Petta differs from Amphictene which has cirri on the lateral and dorsal margins of the operculum, whereas Petta has either a smooth opercular margin or one bearing lappets.
Description: (After Zhang et al. 2019) Cephalic veil completely free from operculum, with smooth or bearing several lappets (slightly raised mounds) anterior margin. Pair of lateral ear-shaped lobes adjacent to dorsal side of cephalic veil. Operculum semicircular with smooth dorsal and lateral margins. Ventral margin of operculum with a transverse row of numerous stout notopodial paleae on each side. Pair of ventral lappets on segment 1. Two pairs of comb-like branchiae on segments 3–4, consisting of large basal hump and series of well separated free lamellae. Pair of dorso-lateral pads on segment 5. Notopodia with paleae on segment 1 and with notochaetae on segments 5–21 (17 pairs). Neuropodia present on segments 8 (or 7) –21, more than 14 pairs with transverse tori, each with a row of uncini. Uncini with circular in cross-section anterior peg ending in a blunt tip, and a posterior longitudinal row of two major teeth. Scaphe distinctly or indistinctly separated from posterior segments, with six pairs of distinct triangular lobes on lateral margins and a vestigial anal flap.