Like many of the Linythiidae, the genus Erigone is small in size, being around 2mm in length. The margin of the carapace is toothed, at least in the males, which also have more robust chelicherae than the female. Identification of the females to species level can require the epigyne be dissected and viewed from within.
The most common species, at least in my garden in Dorset UK, are Erigone dentipalpis and Erigone atra.
Head and chelicerae of male Erigone dentipalpis
Head and chelicerae of female Erigone atra
Carapace of male Erigone dentipalpis, showing "teeth" around the periphery
Pedipalp Erigone atra,
Pedipalp of Erigone atra shown at reduced magnification to indicate the nodules on the metatarsus
Pedipalp of Erigone dentipalpis. There are many similarities with that of Erigone atra, but also distinguishable differences.
One of several variations of the Epigyne of Erigone atra (ventral view)
Dissected dorsal view of the Epigyne of Erigone atra. This does not show all the features of the sketch in Reference 2, Vol. 2, Fig. 43c. In particular, the larger round feature above the two smaller round objects at the bottom centre of the figure, was visible using a light microscope, but is absent in the electron micrograph.