. Bulletin of the British Museum (Natural History), Geology. 162 SMITH, PAUL, GALE & DONOVAN Description. SHAPE AND SIZE. The specimens available for study range 36-60 mm in length (mean = 47mm; SD = 8-3; N = 9), though fragmentary smaller specimens are known. The test height is 56-67% of test length (mean = 60%; SD = 40; N = 9), and the test width 88-105% of test length (mean = 96%; SD = 3 1; N = 9). In outline the test is cordate. The widest part of the test is approximately mid-length, level with genital plates 1 and 4. The test in profile is characteristically subquadrate. The oral sur
. Bulletin of the British Museum (Natural History), Geology. 162 SMITH, PAUL, GALE & DONOVAN Description. SHAPE AND SIZE. The specimens available for study range 36-60 mm in length (mean = 47mm; SD = 8-3; N = 9), though fragmentary smaller specimens are known. The test height is 56-67% of test length (mean = 60%; SD = 40; N = 9), and the test width 88-105% of test length (mean = 96%; SD = 3 1; N = 9). In outline the test is cordate. The widest part of the test is approximately mid-length, level with genital plates 1 and 4. The test in profile is characteristically subquadrate. The oral surface is more or less flat. The anterior face is almost vertical and curves fairly sharply to meet the dorsal surface, which in profile is almost flat. The posterior is sharply truncated and may even be very slightly undercut. The highest point of the test coincides with the apical disc and lies very slightly anterior of the mid-line. Viewed from the anterior, the dorsal surface slopes gently away from the mid-line and the frontal ambulacrum forms a narrow flat zone apically. The frontal groove is well developed and there is a moderately sharp carina on adjacent interambulacral zones. At the ambitus the depth of the frontal groove is 5-8% of the test length (mean = 7%). It is well developed on the anterior surface and continues orally up to the peristome. Adapically, however, it disappears as ambulacrum III curves sharply to face almost vertically upwards, and here lies more or less flush with adjacent interambulacra. The plastron is gently but unmistakably keeled along the mid-line. APICAL DISC. The apical disc is three times as long as it is broad and lies slightly anterior of the mid-line. Its length is 12-15% of the test length, and the anterior edge lies at 30-38% of the test length from the anterior border of the test (mean = 35%; SD = 11; N = 9). The exact arrangement of plates in the apical disc is variable (Fig. 63). Ocular plates II and IV abut along the median suture
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