In this genus ten free somites (mesosoma) occur between the prosomatic and metasomatic carapaces.
40) the anterior and posterior carapaces constitute almost the entire body, the two carapaces being connected by a mid-region of only two free somites.
A, Youngest stage with no mesosomatic somites; B and C, stages with two mesosomatic somites between the prosomatic and telsonic carapaces; D, adult condition, still with only two free mesosomatic somites.
C, Stage with two free mesosomatic somites between the prosomatic and telsonic carapaces.
In none of them are the appendages known, but in the form of the two carapaces and the presence of free somites they are distinctly intermediate between Limulus and the Trilobitae.