The key below is based on Gil (2011), who, in turn, adapted those from Fauchald (1977), Holthe (1986) and Hutchings & Peart (2002), and is also complemented with recent papers such as Nishi et al. (2014), Zhang & Qiu (2017), Nogueira et al. (2019) and Zhang et al. (2019).
Several features concerning morphological changes of uncini related to growth as commented above are included. A formula confronting the number of chaetigers with only notopodia or dorsal chaetae (dc) vs those with biramous parapodia or also with ventral uncini (vu) is included as a key character for discriminating between genera. This is referred mostly for Icelandic specimens because of the many inconsistencies found in the literature across species (see Discussion).
1. Cephalic veil (tentacular membrane) smooth; scaphe indistinctly separated from abdomen; dc/vu = 17/14 (Petta)...................................................................................... Petta pusilla Malmgren, 1866 *
– Cephalic veil cirrate; scaphe distinctly separated from abdomen; dc/vu not 17/14.......................... 2
2. Opercular rim with cirri (serrated) or lappets; dc/vu = 17/13 (Amphictene)................................................................................................................................ Amphictene auricoma (O.F. Müller, 1776)
– Opercular rim smooth; dc/vu = 17/13 or different............................................................................ 3
3. Cephalic veil attached to lateral margin of opercular rim; dc/vu = 16/12 (Lagis)........................................................................................................................................... Lagis koreni Malmgren, 1866
– Cephalic veil free from opercular rim; dc/vu not 16/12.................................................................... 4
4. Uncini with more than one longitudinal row of major teeth in well-developed specimens; dc/vu = 17/13 (Pectinaria).......................................................................... Pectinaria belgica (Pallas, 1766) *
– Uncini with only one longitudinal row of major teeth in well-developed specimens; dc/vu = 17/12 (Cistenides)........................................................................................................................................ 5
5. Paleae short, with blunt tips; all uncini of pectinate type, with one vertical row of teeth, no dorsoventral variation in same unciniger or along the body...................... Cistenides granulata (Linnaeus, 1767)
– Paleae long, with pointed tips; uncini shape avicular type in small and medium-sized specimens showing dorso-ventral variation; pectinate type (single vertical row of teeth) within the same chaetiger and along the body in large ones....................... Cistenides hyperborean Malmgren, 1866