Annals of the South African Museum - Annale van die Suid-Afrikaanse Museum (1980) (14803439683)

Identifier: annalsofsouthaf801980sout (find matches)
Title: Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum
Year: 1898 (1890s)
Authors: South African Museum
Subjects: Natural history
Publisher: Cape Town : The Museum
Contributing Library: Smithsonian Institution Libraries
Digitizing Sponsor: Biodiversity Heritage Library



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found in the False Bay region of Zululand. B-C. SAM-K5889. Juvenile specimen showing pseudotricarinate inner whorls, x 1. different rows through the spUtting off on the ventral side of a new row, thoughstill connected by a common base (Fig. 54A-C). This is the Parabevahites stage.With increasing diameter the inner of these two rows migrates dorsad from thespiral suture to complete the separation of the two rows. This is the Para-texanites stage. In the third stage of development, faint indications of the lateral (2) tuberclestart appearing near the dorsal third of the flanks. The moment of appearanceand the strength of these lateral tubercles are very variable. On the later part of the phragmocone, and on the body chamber, whichconsists of one whorl on the holotype (Figs 50-51), bifurcations cease andribbing becomes single. The whorl section becomes progressively inflatedand towards the last part of the body chamber all lateral ornament weakens. CRETACEOUS FAUNAS FROM SOUTH AFRICA 73
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Fig. 54. Plesiotexanites stangeri (Baily, 1855). A-C. SAM-PCZ5686. Specimen illustrating the Paratexanites stage of ontogeny, but with distinct separated external tubercles. D-F. SAM- K5550. Similar specimen with wavy lateral pseudo-keels. A, C, D-E x 1; B x 2. 74 ANNALS OF THE SOUTH AFRICAN MUSEUM Intraspecific variation Variation in density and prominence of ornament is extensive in thisspecies, and has led to the usage of names such as sparsicostd and densicosta\initially adopted by Spath (1922: 138). These varietal names are applicablemainly to density of ribbing on the inner whorls as shown by Spath (1922,pi. 5 (figs 1-2)) and here as Figures 57, 58. The present material shows thatvirtually all transitions occur from closely ribbed specimens, SAM-K5447(Fig. 59), BMNH-C19443 (Fig. 60), via the holotype of the typical form(Figs 50-51) to the holotype of var. densicosta (Figs 55-57). Apart fromdensity, differences exist in strength of ornament on later parts of the shell.