Named by Carter.
Original description: Test large, conical, pointed apically, with eight or nine post-abdominal chambers and a short, tapering cylindrical horn. Cephalis hemispherical and imperforate with several heavy outer costae penetrating almost on to horn; all remaining chambers trapezoidal, expanding gradually in width as added; final post-abdominal sometimes decreasing in width (see holotype, pl. HSU11, fig. 1). Inner layer of pore frames linearly arranged, square to subrectangular in shape, pores subrounded. Outer layer of test consisting of irregularly twisted costae with small rounded nodes superimposed on circumferential ridges.
Original remarks: Hsuum plectocostatum n. sp. differs from Hsuum optimum Carter in having a more broadly conical shape and the costae, although irregularly fluted/twisted, are more continuous and possess small rounded nodes at ridges. It differs from H. infirmum Sashida (1988) in having a much larger test with a lobate outline, and stronger costae.
Etymology: From the Latin: plecto costatus, -a, -um referring to twisted costae.
Full reference: S. Gorican, E. S. Carter, P. Dumitrica, P. A. Whalen, R. S. Hori, P. De Wever, L. O'Dogherty, A. Matsuoka, and J. Guex. 2006. Catalogue and systematics of Pliensbachian, Toarcian and Aalenian radiolarian genera and species 446
Sister taxa: Hsuum acutum, Hsuum altile, Hsuum amabile, Hsuum arabicum, Hsuum arenaense, Hsuum belliatulum, Hsuum brevicostatum, Hsuum cuestaense, Hsuum feliformis, Hsuum gratum, Hsuum lucidum, Hsuum lupheri, Hsuum matsuokai, Hsuum maxwelli, Hsuum mclaughlini, Hsuum mirabundum, Hsuum modicum, Hsuum mulleri, Hsuum optimum, Hsuum parasolense, Hsuum parvulum, Hsuum robustum, Hsuum rosebudense, Hsuum rutogense, Hsuum tamanense
Type specimen: Holotype GSC 99415. Its type locality is Yakoun River, GSC loc. C-156399, which is in an Aalenian marine carbonate in the Phantom Creek Formation of Canada.
Ecology: passively mobile planktonic omnivore