. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. Fig. 77. Olcostephanus {Olcostephanus) angusticoronatus (Imlay) ($). The holotype from the Taraises Formation of northern Mexico (after Imlay 1938). x 1. venter, whereas the whorl section of O. baini baini (<$) is almost semicircular. Kitchin (1908) considered the weak umbilical tubercles of O. rogersi important in the separation of this species from O. baini but this feature is due to the fact that the holotype is an internal mould. Olcostephanus baini var. sphaeroidalis (Spath) ($) has a less in
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. Fig. 77. Olcostephanus {Olcostephanus) angusticoronatus (Imlay) ($). The holotype from the Taraises Formation of northern Mexico (after Imlay 1938). x 1. venter, whereas the whorl section of O. baini baini (<$) is almost semicircular. Kitchin (1908) considered the weak umbilical tubercles of O. rogersi important in the separation of this species from O. baini but this feature is due to the fact that the holotype is an internal mould. Olcostephanus baini var. sphaeroidalis (Spath) ($) has a less inflated form while the secondary ribbing is also prorsiradiate. Whilst the differences between O. rogersi and O. baini may seem slight, the very marked differences between the macroconch forms of these two species suggests that the separation is a valid one. In describing the ammonite fauna of the Taraises Formation of northern Mexico, Imlay (1938) created the new species Valanginites angusticoronatus (Fig. 77) for very involute, globose forms with rursiradiate primary ribs termi- nating in 16 umbilical bullae from which radiate bundles of 3-4 ribs. There are two prominent parabolae per whorl. This species is very close to the inner whorls of O. rogersi macroconch from which it appears to differ in being somewhat more involute and more finely ribbed. Although originally assigned to Holcostephanus, 'H.' bachelardi Sayn (Fig. 78) has recently been included in the synonymy of Valanginites psaephoides (Mayer-Eymar) (Thieuloy 1911 a). Olcostephanus imbricatus (Baumberger) (Fig. 79) is a strongly inflated, globose species with coarse almost radial secondary ribs which arise from 19-20 umbilical bullae. According to Baumberger (1908: 17), the secondaries arise in pairs from the bullae but quickly bifurcate so that 4 ribs from each bulla cross the venter. There are prominent parabolae on the inner whorls. This species is very close to the macroconch of O. rogersi and this rela
Size: 1432px × 1745px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
