RESUMO
The echinoid genus Tetrapygus was initially described by L. Agassiz (1841) based on a single species, Tetrapygus niger Molina, 1782. Since the extensive work conducted by Mortensen (1935), Tetrapygus has received limited taxonomic attention over the past century. Recent discoveries of new fossil species of Arbacia Gray, 1835 from the upper Pliocene of northern Chile revealed striking morphological similarities between the two distinct Arbaciidae genera Arbacia and Tetrapygus. These findings compelled new investigations to evaluate the taxonomic status of these genera. Based on molecular mitochondrial (COI), nuclear (28S), and morphological evidence, Tetrapygus niger is here recovered as the sister species to Arbacia dufresnii, both species forming a clade within the phylogeny of South American species of Arbacia. Consequently, the diagnosis and description of Tetrapygus niger are here revised, and the species is reattributed to Arbacia, as previously proposed by A. Agassiz in Agassiz & Desor (1846) under the species name Arbacia nigra. An emended diagnosis of Arbacia is also proposed in light of these new findings.