You might like to call all the large Phacus with a long tail just longicauda. But no. There is more to it. Ever since Ehrenberg described the species in 1831 there has been a wide variety of names for different taxa in this group. A group of Polish researchers recently wrote a nice paper about the Phacus longicauda complex. They described 8 different species, that all have a long tail.
There is a nest of Euglenid-experts in Poland. We had the great pleasure to have one of the best ones to be our guest at the annual meeting of the Finnish Phytoplankton Association last spring in Jyväskylä – prof. Bozena Zakrys. Now her student is the first author of this interesting article, where the longicauda comlpex is fixed up. They describe 8 different Phacus species with a long tail. The old buddies, Phacus longicauda (Ehrenberg) Dujardin 1841 of course, but also circumflexus Pochmann 1942, helikoides Pochmann 1942 and tortus (Lemmerman) Skvortzov 1917 get all an amended diagnosis.
They further rise Phacus rotunda (Pochmann) and Phacus cordata (Pochmann) to the species level, both Zakrys et M. Łukomska as authors. The Polish also described two totally new species: Phacus cristatus Zakrys et M. Łukomska and Phacus crassus Zakrys et M. Łukomska.
Only Ph. longicauda and Ph. tortus were on the official Finnish phytoplankton list, now they are accompanied by Ph. rotunda, which was kindly identified from a photo by prof. Zakrys.
Take a look at he nice picture in the article.
Łukomska-Kowalczyk, Maja, Anna Karnkowska, Rafał Milanowski, Łukasz Łach, and Bozena Zakrys (2015). Delimiting species in the Phacus longicauda complex (Euglenida) through morphological and molecular analyses. J. Phycol. 51, 1147–1157.