''Helcionopsis radiatum'' is an extinct species of paleozoic

monoplacophora Monoplacophora , meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell inhabiting deep sea environments . Extant representatives were not recognized as such until 1952; previously they were known only from ...

n in the family

Tryblidiidae Tryblidiidae is an extinct family of paleozoic monoplacophorans in the superfamily Tryblidioidea. Genera Genera in the family Tryblidiidae include: * ''Tryblidium'' Gustaf Lindström, Lindström, 1880. From Ordovician and Silurian.W. H. Dall ...

. This species was first named as ''Tryblidium radiatum'' and described from Silurian in crystalline limestone of Wialmsudd near

Fårösund Fårösund is a locality situated on the Swedish island of Gotland with 800 inhabitants in 2014. The village can be reached by car from Visby. The island of Fårö can be reached by ferry from Fårösund. Fårösund is the northernmost town in th ...

in Sweden by

Gustaf Lindström Gustaf Lindström (27 August 1829 – 16 May 1901) was a Sweden, Swedish paleontologist. He was born in Visby on Gotland. In 1848 he entered Uppsala University, and in 1854 he took his doctor's degree. Having attended a course of lectures in ...

in 1884. Lindsröm found only one specimen.

Shell description

The shell is oval and much elevatedly conical. The greatest height is lying near the middle of the longitudinal axis. The apex is in anterior end, truncated, curved as to lean forwards over the anterior margin of the aperture. The shell between the apex and the margin is consequently much concave. The apertural border is straight and horizontal. The surface is covered by fine, closely packed, radiating striae and a few concentric lines of growth. The shell has been thick, as may be seen by a few fragments which are left along the borders. The length of the shell is 24 mm, width 20 mm and the height of the shell is 15 mm. Length of the aperture is 23 mm.

References

This article incorporates public domain text from reference. {{Taxonbar, from=Q5701570 Prehistoric monoplacophorans Molluscs described in 1884