Rachiosoma irregulare Fourtau, 1921

Mohamed Abdelhamid, Marouf Abdel-Aty & Moustafa Azab, Mahmoud, 2012, Turonian-Santonian echinoids from Egypt, Geodiversitas 34 (3), pp. 575-615 : 590

Rachiosoma irregulare Fourtau, 1921 ( Figs 7G ; 8E, F)

Rachiosoma irregulare Fourtau, 1921: 39 , pl. 5, figs3, 4. — Abdelhamid & El Qot 2001: 18, fig. 5D, E. — El Qot 2006: 137, pl. 31, figs 8, 10a, b.

MATERIAL EXAMINED. — Three specimens from the Turonian of Wadi Abu Qaada (Wata Formation), AS- UAQE 5-7, Wadi Abu Qaada echinoid horizon 4 ( QEH 4).

DESCRIPTION

Test diameter attaining 15.5-24.6 mm. Adapical surface convex, high (H/D = 0.47-0.54). Outline of apical disc pentagonal, caducous ( Fig. 8E) (La/D = 0.43-0.47). Ambulacral area wide (Wa/Wi = 0.62- 0.75). Poriferous zones uniserial. Major ambulacral plates quadrigeminate, but they are trigeminate in three plates adjacent to peristome. Two columns of granules occur around the perradial suture. Interambulacral area with two columns of imperforate,crenulated, and nonconfluent primary tubercles. Adradial extrascrobicular surface wide and occupied by very small tubercles ( Fig.7G ). Interradial extrascrobicular surface occupied by two columns of smaller granules.

REMARKS

This species is distinguished from R. rectilineatum (Péron& Gauthier, 1881) in having wider ambulacra at the apex, well developed ambulacral tubercles on adapical surface, relatively higher test (H/D = 0.47-0.54 against 0.38-0.49 in R. rectilineatum ), and the mamelonated granules on the adradial extrascrobicular surface reach both the apical disc and peristome whereas in R. rectilineatum they are more developed but confined to the ambital area.

Figures

FIG. 7. — Camera lucida drawings: A, B, plating, tuberculation and poring in Phymosoma baylei (Cotteau, 1864) from the Turonian of Wadi Abu Qaada; A, ambital ambulacral plates; B, ambital interambulacral plate; C, D, plating, tuberculation and poring in Phymosoma beadnelli (Gregory, 1906) from the Coniacian-Santonian of Abu Roash; C, adapical ambulacral plates; D, ambital interambulacral plates; E, F, plating, tuberculation and poring in Phymosoma thevestense (Péron & Gauthier, 1879) from the Coniacian-Santonian of Abu Roash; E, ambital interambulacral plates; F, subambital ambulacral plates; G, plating and tuberculation of ambital interambulacral plates in Rachiosoma irregulare Fourtau, 1921 from the Turonian of Wadi Abu Qaada. Scale bars: A, 0.8 mm; B, 0.9 mm; C, D, 1 mm; E, F, 0.5 mm; G, 0.7 mm.