I have to admit to becoming increasingly glad that I don't work on mosses. The new Taxon of the Week is a moss, and looking up stuff on it has driven me into a strange world of unfamiliar terminology and fine-scale features. If I mess anything up here, I only hope that the legions of moss fans out there* forgive my transgressions.
*Do not doubt that they're out there. As I've commented before, bryologists are a dedicated bunch.
Ectropothecium is a genus of mostly hydrophytic mosses found worldwide (hydrophytic plants grow either in water or in completely waterlogged soil; some Ectropothecium species do the former, others the latter). It belongs to a clade of mosses known as the pleurocarpous mosses; while other (acrocarpous) mosses branch only rarely and produce terminal archegonia (the female reproductive organs - see the diagram at the posts linked to above) at the end of the stem, pleurocarpous mosses produce lateral archegonia on highly branched and extensively interwoven stems (Shaw & Renzaglia, 2004). Pleurocarpous mosses are divided molecularly into three orders, Ptychomniales, Hookeriales and Hypnales; Ectropothecium belongs to the Hypnales which have a smooth spore capsule with a calyptra (protective cap) that usually opens by splitting along one side (Buck et al., 2004). Within the Hypnales, Ectropothecium is placed in the family Hypnaceae; however, phylogenetic studies of Hypnales (e.g. De Luna et al., 2000) suggest that members of the Hypnaceae may be para- or polyphyletically placed within the order.
Ectropothecium itself is distinguished by having spore capsules that are very small (usually less than 1 mm long) and almost spherical (Buck & Tan, 2008), non-decurrent leaves (not extending down the stem where they join), short and broad leaf cells and filamentous pseudoparaphyllia that are two or three cells wide at the base (Ireland, 1992). Pseudoparaphyllia are small outgrowths of the stem that cluster around the base of new side-branches (as opposed to paraphyllia which are normally scattered more evenly along the entire stem); they may be thread- or blade-shaped. See Ignatov & Hedenäs (2007) for a review of the distinctions between paraphyllia, pseudoparaphyllia and proximal branch leaves, though to be honest if you understand the difference you're somewhat ahead of me (as far as I can tell, pseudoparaphyllia grow on the stem around the primordium of a new branch and remain on the stem, while proximal branch leaves may start growing around the primordium but end up being transferred to the new branch).
Buck, W. R., C. J. Cox, A. J. Shaw & B. Goffinet. 2004. Ordinal relationships of pleurocarpous mosses, with special emphasis on the Hookeriales. Systematics and Biodiversity 2 (2): 121-145.
Buck, W. R., & B. C. Tan. 2008. A review of Elmeriobryum (Hypnaceae). Telopea 12 (2): 251-256.
De Luna, E., W. R. Buck, H. Akiyama, T. Arikawa, H. Tsubota, D. González, A. E. Newton & A. J. Shaw. 2000. Ordinal phylogeny within the hypnobryalean pleurocarpous mosses inferred from cladistic analyses of three chloroplast DNA sequence data sets: trnL-F, rps4, and rbcL. Bryologist 103 (2): 242-256.
Ignatov, M. S., & L. Hedenäs. 2007. Homologies of stem structures in pleurocarpous mosses, especially of pseudoparaphyllia and similar structures. In Pleurocarpous Mosses: systematics and evolution (A. E. Newton & R. Tangney, eds) pp. 227-245. The Systematics Association Special Volume Series 71. Taylor& Francis / CRC Press: Boca Raton.
Ireland, R. R. 1992. Studies of the genus Plagiothecium in Australasia. Bryologist 95 (2): 221-224.
Shaw, J., & K. Renzaglia. 2004. Phylogeny and diversification of bryophytes. American Journal of Botany 91 (10): 1557-1581.