Silurian calymenid trilobite Alcymene n. gen., with new species from the Ludlow of Gotland, Sweden

Alcymene n. gen. comprises a monophyletic group of calymenids known from the Ludlow Series of Sweden and the British Isles. The sister species Alcymene alveus and A. lindstroemi n. gen. and spp. (Hemse Beds, Gotland, Sweden) are most closely related to the type species A. neointermedia (Richter and Richter), which occurs in overlying and underlying strata at nearby localities. Structural modifications of A. alveus and A. lindstroemi, especially the development of a greatly broadened and troughed librigenal border, can be interpreted as "snowshoe' adaptations for increased stability on an offshore mud substrate.