. Bulletin of the Natural Histort Museum. Geology series. 54 HOWARTH. Fig. 2 Pachysphinctes bathyplocus (Waagenj, m above base of lower marly part of Kilya Member (fauna 7), Naifa Cliff; CA995, ending as very large wedge-shaped radial ridges at the largest sizes, so that rib densities as low as 18 primary ribs per whorl occur at 400 mm diameter. When the primary ribs start to increase in size and strength, three to five secondary ribs are associated with each primary, either by multiple splitting or intercalation. As size increases the association between primaries and secondar
. Bulletin of the Natural Histort Museum. Geology series. 54 HOWARTH. Fig. 2 Pachysphinctes bathyplocus (Waagenj, m above base of lower marly part of Kilya Member (fauna 7), Naifa Cliff; CA995, ending as very large wedge-shaped radial ridges at the largest sizes, so that rib densities as low as 18 primary ribs per whorl occur at 400 mm diameter. When the primary ribs start to increase in size and strength, three to five secondary ribs are associated with each primary, either by multiple splitting or intercalation. As size increases the association between primaries and secondaries becomes less clear, then the secondaries gradually disappear so that the venter becomes broad and smooth from 225-250 mm diameter. One to three oblique constrictions on inner whorls tend to disappear beyond 150 mm diameter. Measurements. In ammonites it is traditional to measure whorl breadth in the inter-costal space between ribs, rather than across the tops of the ribs themselves. In a species like P. major that has very heavy ribs at large sizes, such a measurement of whorl breadth makes the whorls appear much less depressed than when the meas- urement is taken across the tops of the ribs. In the measurements below an extra figure is given for the whorl breadth across the tops of the ribs (WbR) which better illustrates the real amount of whorl depression. D Wh Wb WbR U Lectotype () — () () (from Waagen) Lectotype () — () () (from pi. 54) CA1170 () () () () CA1170 () () () — CA1171 () () () () CA998 () () () — CA1005 () () () () CA1157 () () () () CA1014 () () — () Remarks. The largest specimen figured hitherto is Waagen's (1875: pi. 54) lectotype, which i
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