Peribrissus Pomel, 1869

Holmes, Francis C., 2011, A new species of Peribrissus (Echinoidea, Spatangoida) from the middle Miocene of South Australia, Memoirs of Museum Victoria 68, pp. 29-35 : 30-31

Genus Peribrissus Pomel, 1869

Type species. Peribrissus saheliensis Pomel, 1883 , by subsequent monotypy.

Other species. P. sotgiai Giorgio, 1923 .

Diagnosis. Modified from Smith et al. (2005). Test medium to large and cordiform with distinct anterior sulcus, posterior face oblique to vertically truncate, profile depressed to moderately domed. Apical disk well anterior of centre, ethmolytic with three gonopores. Ambulacrum III sunken aborally, the groove increasing in width and depth to ambitus, with rows of enlarged tubercules occurring just outside adradial sutures, pores small. Petals straight, narrow and depressed, cruciform, the anterior pair longer than the posterior pair. Peristome and plastron plating of type species unknown. Periproct high on posterior truncate face. Semipetalous fasciole band combines with continuous marginal fasciole immediately behind and below anterior petals .

Remarks. There has been confusion regarding the designation of the type species of Peribrissus . Fischer (1966, p. U576) and Smith et al. (2005) stated that P. saheliensis is the type species by original designation, but Pomel (1869, p. 13) did not name any species he assigned to his genus, for which he gave only a very brief diagnosis and made comparisons with Prenaster . Pomel later (1883, p. 36) gave a slightly more detailed diagnosis followed by the statement ‘ P. saheliensis est du miocène supérieur’. As saheliensis was the only named species assigned to Peribrissus , the diagnosis given for the genus applies also to the species, thus satisfying the criteria for availability (ICZN, Article 12.2.6) and making P. saheliensis the type species by subsequent monotypy (ICZN, Article 68.3).

Two species of Pericosmus described by McNamara and Philip (1964) from the Miocene of Australia — P. celsus and P. quasimodo — were reassigned by Smith et al. (2005) to Peribrissus . Though Pericosmus and Peribrissus are superficially alike, the path of the peripetalous fasciole in the two Australian species is clearly different from that in the Prenasteridae and, consequently, in Peribrissus . Smith et al. (2005) stated that in the Prenasteridae , ‘marginal and peripetalous fasciole combine anteriorly, the combined band passing several plates below the end of the anterior petals’. In contrast, the peripetalous fasciole in Pericosmus celsus and P. quasimodo — as well as in P. torus , also erected by McNamara and Philip in the same paper — follow a distinctly different path. These three species have the peripetalous fasciole closely bounding the distal end of the anterior petals, then transversely crossing interambulacral plates in columns 2a and 3b before taking a longitudinal path (sometimes irregular and/or intermittent) towards the marginal fasciole in interambulacral columns 2b and 3a. Due to the state of preservation of the numerous Pericosmus specimens inspected in Museum Victoria and private collections, it is not possible to determine whether the peripetalous fasciole always reaches the marginal fasciole on either side of the anterior sulcus. Nevertheless, in all Australian species assigned to Pericosmus , including P. compressus Duncan, 1877 and P. maccoyi Gregory, 1890 , the peripetalous fasciole closely bounds the distal end of the anterior petals and continues transversely onto interambulacra 2 and 3, clearly negating any reassignment to Peribrissus. However, whether the five Australian fossil species listed above strictly belong in the genus Pericosmus is a matter of conjecture, considering the type species Pericosmus latus Desor in Agassiz and Desor, 1847, has separate and continuous marginal and peripetalous fascioles, the latter crossing ambulacrum III well above the anterior margin.

Stefanini (1911, p. 86) reassigned Prenaster excentricus ( Wright, 1855) to Peribrissus in the belief that the two genera overlap based on the similarity of their upper test profile with highly eccentric anterior apex and four ethmolitic genital pores. Pomel’s statement (1887, p. 63) — that the number of genital pores in Peribrissus is unknown — seems to have been ignored by Stefanini, whose reference to four genital pores may have been based on details of Wright’s species. Giorgio (1923, p. 125), in describing Peribrissus sotgiai from Sardinia, accepted Stefanini’s finding that Wright’s Prenaster from Malta was a Peribrissus ; noting that P. sotgiai has four gonopores, but that the right anterior one is poor and almost atrophied. These statements appear to have resulted in Mortensen (1951) and Fischer (1966) listing both genera as having four genital pores. However, of the eight genera now included in the family Prenasteridae by Smith et al. (2005), only Peribrissus and Tripylus Philippi, 1845 are listed as having three genital pores. Although both of these genera have a well-defined anterior sulcus, Peribrissus is easily distinguished from Tripylus by the markedly anterior location of its apical disk compared to the central position in the latter. The lack of a sulcus and the presence of four genital pores in species of Prenaster clearly refute Stefanini’s reassignment of Prenaster excentricus to Peribrissus .