Cylindroiulus pyrenaicus is a distinctive species since it bears both a projecting telson and ventral scale.

 

In Britain and Ireland, this combination of characters is otherwise only seen in C. apenninorum and Enataiulus armatus.  The former is much larger, reaching 30mm or more in length, and the latter has conspicuous fringing setae on the metazonites.  The male gonopods are characteristic.

 

A description, with figures, is given by Gregory, Owen, Jones & Williams (2018), who also note the occurence of the ectoparasitic fungus Rickia laboulbenioides.