The Echinoid Directory

Plagiobrissus Pomel, 1883, p. 29

[=Plagionotus Agassiz & Desor, 1847, p. 119 non Mulsant, 1842; =Plagiostoma d'Orbigny, 1854, p. 151, non Sowerby, 1812 ]

Diagnostic Features
  • Test ovate with or without shallow anterior sulcus. Depressed in profile.
  • Apical disc ethmolytic, with 4 gonopores.
  • Anterior ambulacrum narrow and weakly sunken; pore-pairs small, simple isopores; plates almost as tall as wide adapically.
  • Other ambulacra narrow and sunken. The anterior pair forming an obtuse angle, the posterior flexed outwards distally. Petals parallel-sided with little perradial space between pore-pair columns. Distal plates in petals occluded.
  • Periproct large, on short, slightly undercut, truncate face.
  • Peristome wider than long; kidney-shaped; facing obliquely forwards.
  • Labral plate short and wide and in broad contact with both sternal plates. Sternal plates long and narrow; parallel-sided. Episternal plates large and paired, about 2/3rds length of sternal plates and forming posterior part of plastron. Sternal-episternal suture concave to rear. Post episternal plates paired.
  • Posterior ambulacral zones on oral surface subparallel and narrow. Ambulacral plate 6 expanded to enter the subanal fasciole.
  • Primary spines and tubercles common within peripetalous fasciole; forming horizontal rows on Iamb plates; tubercles not sunken. Plastron fully tuberculate.
  • Well-developed subanal fasciole and peripetalous faciole. Subanal fasciole shield-shaped and with anal branches; enclosing four tube-feet on either side.
Upper Eocene to Recent, Europe, North America and the Caribbean.
Name gender masculine
Echinus grandis Gmelin, 1788, p. 320, by original designation.
Species Included
  • P. grandis (Gmelin, 1788); Recent, Gulf and Caribbean.
  • P. africanus (Verrill, 1871); Recent, West African coast.
  • P. jacksoni (Lambert & Sanchez Roig, 1924); Oligo-Miocene, Cuba.
  • P. dixoe (Cooke, 1942); Upper Eocene, USA.
Classification and/or Status
Spatangoida, Micrasterina, Brissidae.

Distinguished from Metalia by having large primary tubercles within the peripetalous fasciole. Some Macropneustes come close in aboral appearance, but the oral plastron plating is very different, without having the paired episternals and post-episternals or shield-shaped subanal fasciole. Rhabdobrissus was established as a subgenus of Plagiobrissus for forms without a frontal groove. Whether Rhabdobrissus really turns out to be a monophyletic subset within Plagiobrissus, a paraphyletic assemblage or a discrete genus requires a modern gene-based study of the clade.  For the moment it is retained as a separate taxon.

Mortensen, T. 1951. A monograph of the Echinoidea. V. Spatangoida 2. C. A. Reitzel, Copenhagen.

Pomel, A. 1883. Classification méthodique et genera des Échinides vivante et fossiles. Thèses présentées a la Faculté des Sciences de Paris pour obtenir le Grade de Docteur ès Sciences Naturelles, 503, 131 pp. Aldolphe Jourdan, Alger.