The Echinoid Directory

• Test subglobular, flattened below with slightly sunken peristome.
• Apical disc small, less than one-third test diameter; caducous, plating unknown in type species.
• Ambulacra straight and about half the width of interambulacra. Pore-pairs uniserial to peristome, undifferentiated.
• Ambulacral plating trigeminate, with echinid-style compounding throughout. Lowest element the largest, middle element a small demiplate and upper element narrow but extending to perradius.
• Ambulacral plates with a distinct primary tubercles close to the pore zone and a smaller secondary towards the perradius; primary tubercles separated in column by a row of miliaries; similar in size to interambulacral primary tubercles.
• Interambulacral plates with central primary tubercle and adradial and interradial zones of secondaries and miliaries closely packed; on ambital and adoral plates a larger secondary flanks either side of the primary.
• Primary tubercles imperforate and non-crenulate, the mamelon relatively large, and the areole distinctly incised.
• No true pits on test but small gaps between the raised tubercles along sutures give a slightly sculpted appearance.
• Peristome small, circular, with only very feeble buccal notches and no tag.
• No primordial interambulacral plate; no sphaeridial pits.
• Spines and lantern unknown.

Miocene-Pliocene, Europe, North Africa.

Echinus monilis Desmarest, in Defrance, 1816, by original designation.

• A. monilis (Desmarest, in Defrance, 1836); Upper Miocene, France.
• A. catenata (Desor, in Agassiz & Desor, 1846); Middle Miocene. (Burdigalian-Langhian), France, Egypt.
• A. fourtaui (Lambert, 1907); Burdigalian, Egypt.
• A. massylea Pomel, 1887; Helvetian, Algeria.
• A. nicaisei Pomel, 1887; Pliocene, Algeria [probably the same as A. massylea].
• A. fraasi Gauthier. 1901; Miocene, Egypt.
• A. piae Lovisato, 1895; Langian, Italy.
• A. romana (Merian, in Desor, 1858); Pliocene, Sicily

Euechinoidea, Camarodonta; Temnopleuroida, Trigonocidaridae.

Subjective junior synonym of Genocidaris Agassiz, 1869.

The type species is problematic in that it lacks well defined sculturing and has only rather indistinct pits. Its affinities to Temnopleuridae is therefore not certain. Nor is it clear that many of the species currently assigned to this genus truly belong here. Several species assigned to Arbacina do show classic temnopleurid sculpturing, with deep pitting under the primary tubercles. Forms such as A. savini Lambert, 1910, without pitting, seem like good Psammechinus.  A major revision of all the species previously assigned to this genus is required.

Mortensen (1943) separated Arbacina from Genocidaris stating that the former had "tubercles not indented, depressions on horizontal sutures present" while the latter had " tubercles indented, depressions in the horizontal sutures (usually) lacking". Nevertheless, the type species of both genera have almost identical ornamentation, with dense secondary tuberculation and small pits confined to immediately beneath the primary interambulacral tubercles.

Mortensen, T. 1943. A monograph of the Echinoidea Part III.2, Camarodonta 1. C. A. Reitzel, Copenhagen.

Borghi, E., Campos, I. B. & Garcia, A. R. 2005. Arbacina romana (Merian, 1858) from the lower Pleistocene of Favignana Island (Sicily). Parva Naturalia 7, 47-71.

Pomel, A. 1869. Revue des échinodermes et de leur classification pour servir d'introduction à l'étude des fossiles. Deyrolle, Paris, 67 pp.