Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum . flated and the whorl section morerounded (Fig. 72B). SAS-Z 1790a has the inner whorls preserved (Figs 73-74), permitting astudy of the ontogeny. The innermost whorls are completely smooth up to adiameter of c. 8 mm; thereafter ventrolateral nodes start appearing in theumbilical seam, and soon broad, flat ribs appear, some with incipient a diameter of 20 mm umbilical tubercles start appearing at the dorsal endsof these ribs. Ribbing is single throughout. Lateral (2) tubercles appear at adiameter of c. 50 mm.
Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum . flated and the whorl section morerounded (Fig. 72B). SAS-Z 1790a has the inner whorls preserved (Figs 73-74), permitting astudy of the ontogeny. The innermost whorls are completely smooth up to adiameter of c. 8 mm; thereafter ventrolateral nodes start appearing in theumbilical seam, and soon broad, flat ribs appear, some with incipient a diameter of 20 mm umbilical tubercles start appearing at the dorsal endsof these ribs. Ribbing is single throughout. Lateral (2) tubercles appear at adiameter of c. 50 mm. Simultaneously, the ventrolateral (3) tubercles becomeless spinose, and migrate away from the umbilical suture to reveal a fourth rowof tubercles. With increasing diameter, ribbing becomes stronger, rursiradiate andslightly sinuous, and tuberculation more prominent. Ribbing continuesobliquely over the rounded umbilical wall, but very weakly. The umbilicaltubercles (1) are situated high up on the umbilical wall, and tend to migrate up 90 ANNALS OF THE SOUTH AFRICAN MUSEUM. Fig. 70. Plesiotexanites collignoniforme sp. nov. Holotype SAS-Z1789. x 0,6. the ventral part of the flanks. The lateral (2) tubercles are situated near the dorsalthird of the flanks, and, though weakly developed, appear most prominent dueto the strong development of ribbing in this area. The submarginal (3), margi-nal (4) and external (5) rows of tubercles are situated closer to each other thanthe first two rows (1 & 2). The marginal and external tubercles tend to be con-nected by a thickened rib in Paratexanites fashion. The intertubercular area onthe venter is connected by a swelling, and the faint, undulating keel tends toform a slight tubercle in this area. The external tubercles tend to be connected CRETACEOUS FAUNAS FROM SOUTH AFRICA 91
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Keywords: ., bookcentury1800, bookdecade1890, booki, booksubjectnaturalhistory
