. Bulletin of the British Museum (Natural History), Geology. 92 SMITH, PAUL, GALE & DONOVAN APICAL SYSTEM. The plates of the apical disc are firmly bound to the corona and are usually preserved in place. The apical disc is small, only 16-24% of the test diameter (mean = 20%; SD = 2-9; N = 12), and circular in outline. The periproct is centrally positioned and irregularly oval in outline (PI. 13, fig. 2). It is 8-11% of the test diameter (mean = 10%; SD = 10; N = 9). Ocular plates are all exsert and protrude slightly beyond the genital plates. They are covered in miliary tubercles and no oc
. Bulletin of the British Museum (Natural History), Geology. 92 SMITH, PAUL, GALE & DONOVAN APICAL SYSTEM. The plates of the apical disc are firmly bound to the corona and are usually preserved in place. The apical disc is small, only 16-24% of the test diameter (mean = 20%; SD = 2-9; N = 12), and circular in outline. The periproct is centrally positioned and irregularly oval in outline (PI. 13, fig. 2). It is 8-11% of the test diameter (mean = 10%; SD = 10; N = 9). Ocular plates are all exsert and protrude slightly beyond the genital plates. They are covered in miliary tubercles and no ocular pore is visible from above. Genital plates are larger than the ocular plates, the madreporite being noticeably larger than the other four. Each is pierced by a large, centrally-placed gonopore; on the madreporite the gonopore is displaced towards the outer edge to accommodate the madreporic pores. Genital plates come to a small point interradially along their outer edge. Like the ocular plates, genital plates are covered in small miliary tubercles. PERISTOME. This is circular in outline and forms 38-48% of the test diameter (mean = 45%; SD = 30; N = 11). It is more or less flush with the base of the test. Buccal slits are present but are only feebly developed. AMBULACRA. These are straight and very slightly raised. At the apex they are about as broad as the interambulacra but at the ambitus they are only half as broad as the inter- ambulacral zones. At the ambitus they are 16-21% of the test diameter (mean = 18%; SD = 1-6; N = 13). There are 18 compound ambulacral plates per column (54 ambulacral pores) at about 10 mm test diameter, rising to 31 compound plates (94 ambulacral pores) at about 28 mm test diameter. Each plate is broader than it is tall and is compound in the acrosaleniid style (Fig. 33c). Ambulacral pores, which are P2 type (see Smith 1978), are linearly arranged. The pore zone is slightly sunken and forms 20-30% of the plate width at the ambitus. In the larg
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