Original description: Test small, subconical, with short horn. Cephalis and thorax sparsely perforated, covered with layer of microgranular silica. Abdomen and subsequent five to six post-abdominal chambers trapezoidal to rectangular in outline, comprised of an inner latticed layer of small tetragonal pore frames. Outer layer comprised of long, relatively massive costae extending longitudinally from abdomen to final post-abdominal chamber Early chambers increasing in width gradually, first or second post abdominal chamber increasing rapidly in width, subsequent chambers decreasing slightly in width as added.

Original remarks: Hsuum parvulum n. sp. differs from Hsuum gratum n. sp. by having a smaller test with a shorter horn and by lacking ridges on the apical portion of the test.

Etymology: Parvulus-a-um (Latin, adj.) very small.

Hsuum parvulum was named by Yeh (1987). Its type specimen is NMNS (000089) and is a 3D body fossil. Its type locality is Suplee-Izee area, OR-600A, which is in a Toarcian marine tuff/limestone in the Hyde Formation of Oregon.